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Skin & hair

Dark Vertical Line on a Fingernail: Should You Be Worried?

A dark vertical streak running from the base of a nail to its tip is called melanonychia. Most causes are benign — nail injury, a benign matrix lesion, normal pigmentation in darker skin, or medications — but subungual melanoma can look identical, so any new, widening, or changing streak warrants dermatologist evaluation.

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What is melanonychia, and why does a dark band appear in the nail?

Melanonychia means dark pigment — melanin — deposited in the nail plate. The nail grows from a structure at its base called the nail matrix, which contains pigment-producing cells (melanocytes). When those cells become more active or increase in number, they deposit melanin into the growing nail. The result is a colored band that moves upward and outward as the nail grows 1.

This can happen through two distinct mechanisms: *melanocytic activation* (the same number of cells produce more pigment, triggered by trauma, medications, or systemic changes) or *melanocytic hyperplasia* (the cell count itself increases, as with a nail matrix nevus or, in the most serious case, melanoma) 1.

Melanonychia is extremely common in people with darker skin. Studies show it is present in approximately 77% of Black individuals by age 20, and in nearly all Black individuals over age 50 1. In those populations, multiple nails may be involved and the finding is physiologically normal. A new or changing single-nail streak at any age, however, warrants clinical assessment regardless of skin tone.

What are the most common benign causes?

The majority of dark nail streaks are benign. A 2025 clinical review by an international nail expert group, published in the *Journal of the American Academy of Dermatology*, provides the most current framework for understanding the full range of causes 2.

Physiologic or racial melanonychia. In people of African, South Asian, East Asian, or Hispanic descent, melanonychia in multiple nails is a well-recognized normal variant. It is caused by chronic low-level melanocytic activation rather than disease.

Nail matrix nevus. A benign cluster of pigmented cells in the nail matrix — essentially a mole under the nail. This is one of the most frequent causes in younger people, including children, where longitudinal melanonychia in a single nail is usually due to a nail matrix nevus 3.

Post-traumatic pigmentation. A bump, door-jam injury, or repeated friction can activate nail matrix melanocytes. The streak may fade or grow out over several nail growth cycles.

Medication-related melanonychia. Several drugs are documented causes, including hydroxyurea, zidovudine (AZT), minocycline, certain antimalarials, and several chemotherapy agents such as cyclophosphamide and doxorubicin 4. Medication-related streaks often involve multiple nails and resolve after the drug is stopped.

What is subungual melanoma, and why does it matter here?

Subungual melanoma is a cancer arising from the pigment cells of the nail matrix. It represents approximately 0.7–3.5% of melanomas overall, but its importance is disproportionate for two reasons 5.

First, it accounts for a much higher *proportion* of melanomas in people with darker skin. Across several studies, subungual and other acral melanomas represent up to 75% of melanomas in people of African descent, approximately 25% in Chinese populations, and roughly 10% in Japanese populations 56. For a person with darker skin, the nail unit is among the most important sites to monitor — yet awareness of this risk is often low.

Second, subungual melanoma is frequently diagnosed late. Because the streak is often indistinguishable from a benign cause at first appearance, and because the nail obscures direct visualization of the lesion, an average diagnostic delay of approximately two years has been reported in some case series 1. Late-stage diagnosis is directly associated with poorer outcomes.

Up to one-third of subungual melanomas are amelanotic — they produce little or no pigment — which further complicates visual recognition 5.

What is Hutchinson's sign, and how reliable is it?

Hutchinson's sign refers to pigment from the nail band spreading onto the surrounding skin — the cuticle, proximal nail fold, or lateral nail fold. When present, it substantially raises concern for melanoma.

A 2018 study in *JAMA Dermatology* developed a dermoscopic scoring model for subungual melanoma in situ that included Hutchinson's sign among six key features; the sign carried an odds ratio of 18.18 for malignancy 7. The full model (including asymmetry, border fading, multicolor pigmentation, band width, and Hutchinson's sign) achieved an area under the curve of 0.91, with 89% sensitivity and 62% specificity.

However, Hutchinson's sign is not specific to melanoma. Benign causes including Laugier-Hunziker syndrome, Peutz-Jeghers syndrome, racial pigmentation, and traumatic changes can produce a similar appearance — sometimes called a "pseudo-Hutchinson sign" 58. Approximately 30% of benign lesions show some degree of periungual pigmentation 5. The sign raises concern but must be interpreted in clinical context; it neither confirms nor excludes melanoma on its own.

How does a dermatologist evaluate a dark nail streak?

A dermatologist will begin with a clinical history and a careful visual examination, including an assessment of all nails. The single most useful office tool is dermoscopy (also called onychoscopy when applied to nails): a handheld device with polarized magnified light that reveals pigment patterns invisible to the naked eye.

The ABCDEF criteria for nail melanoma guide the clinical assessment 59: - A — Age (peak incidence: fifth to seventh decade) and Ancestry (African, Asian, Hispanic, or Native American background) - B — Brown-black Band with irregular borders or width greater than 3 mm - C — Change in the streak's size, color, or rate of growth - D — Digit most commonly affected (thumb, index finger, great toe) - E — Extension of pigment onto the periungual skin (Hutchinson's sign) - F — Family or personal history of melanoma

It is worth noting that in a retrospective study of 84 biopsied nail lesions, the raw count of ABCDEF criteria met did not differ significantly between melanoma and benign cases, though band width exceeding 40% of the nail plate width was a practically meaningful threshold for biopsy consideration 9. Dermoscopy adds important qualitative information beyond counting criteria.

If dermoscopy findings are suspicious or inconclusive, a nail matrix biopsy provides the definitive answer. The biopsy removes a small sample from the matrix; it is an outpatient procedure, though it requires careful technique because the matrix is also responsible for normal nail plate formation. For low-suspicion lesions that appear benign on dermoscopy, photographic monitoring at defined intervals is an accepted alternative to immediate biopsy 2.

Does age change how a dark nail streak should be interpreted?

Age matters significantly in both directions.

In children, longitudinal melanonychia is most commonly caused by a nail matrix nevus. Nail unit melanoma in children is exceptionally rare. A 2026 review of pediatric cases concluded that most can be safely managed with clinical and dermoscopic follow-up rather than immediate biopsy, reserving surgery for lesions with high-risk features 3. Nail matrix procedures in young children carry a real risk of permanent nail dystrophy, which weighs against routine biopsy of stable, low-suspicion lesions.

In adults over 50, the calculus shifts. A new dark streak in a single nail in a middle-aged or older adult carries a meaningfully higher index of suspicion than the same finding in a teenager. The ABCDEF criteria reflect this: age in the fifth to seventh decade is itself a risk factor. Prompt dermatologic evaluation — not watchful waiting — is the appropriate default for new nail streaks in this age group.

What should you do right now?

Take a clear, well-lit photograph of the nail today. This creates a baseline for comparison.

Book an appointment with a dermatologist rather than waiting to see whether the streak grows out. The evaluation is straightforward, the dermoscopy takes minutes, and the reassurance of a normal finding has real value. If you notice the streak widening, the pigment spreading to the surrounding skin, or the nail becoming crumbly or distorted before your appointment, seek an earlier slot.

Bring to your appointment: - The dated photograph taken today (and any older photos if you have them) - A complete list of current medications and supplements - Notes on any nail trauma in the past year, including minor incidents - Your personal and family history of melanoma or skin cancer

Common questions

Is a dark line on my fingernail always serious?

No. Most dark nail streaks are benign — caused by a healed nail injury, physiologic pigmentation (very common in people with darker skin tones), a nail matrix nevus, or a medication. However, because subungual melanoma can look identical to a benign streak early on, any new or changing streak deserves a clinical evaluation rather than home monitoring alone.

Can I tell at home whether my dark nail streak is cancerous?

No reliable home assessment exists. Features that increase concern — a widening streak, pigment spreading to the cuticle or surrounding skin, nail plate distortion, rapid change — warrant urgent dermatology care rather than continued observation. But the absence of those features does not rule out melanoma. Dermoscopy by a trained clinician is the appropriate first step.

Does having darker skin mean I am at lower risk of nail melanoma?

Not exactly. People with darker skin have a lower overall risk of skin melanoma, but subungual and other acral melanomas are proportionally far more common in darker-skinned populations. For a person of African, Asian, or Hispanic descent, the nail unit is one of the most important sites for melanoma vigilance.

What does a nail matrix biopsy involve?

A nail matrix biopsy is an outpatient procedure performed by a dermatologist or dermatologic surgeon. A small sample of tissue is taken from the nail matrix — the growth zone at the base of the nail. Local anesthesia is used. There is a small risk of permanent nail dystrophy (change in nail shape or texture), which is why biopsy is reserved for lesions where the dermoscopy is suspicious or inconclusive, not performed on every dark streak.

My dark nail streak has been there for years and has not changed. Do I still need to see a doctor?

A long-stable streak with no recent change is reassuring, but clinical evaluation is still worthwhile if you have never had it assessed. A dermatologist can confirm it is benign and establish a documented baseline, which is valuable for detecting any future change.

Talk to a clinician

Nina Osei, NP — Nurse Practitioner

checkups, refills & skin. Gale can match you with a licensed clinician for a visit.

Find care →

When to seek prompt care

  • —Pigment from the nail band spreading onto the surrounding cuticle or skin (Hutchinson's sign)
  • —A streak that is rapidly widening or darkening over weeks to months
  • —The nail plate is crumbling, lifting, or splitting at the base
  • —A new dark streak in a single nail in an adult over 50, especially affecting the thumb, index finger, or great toe
  • —Personal history of melanoma and any new nail pigmentation change
  • —An amelanotic (not pigmented) nodule or growth under or at the nail edge

This article is general health information and does not constitute a diagnosis or personalized medical advice. Only a licensed clinician who has examined you — including with dermoscopy — can appropriately assess a dark nail streak.

References

  1. 1.Metzner MJ, Billington AR, Payne WG (2015). Melanonychia. Eplasty. PMID 26335443 ✓Epidemiology of melanonychia in Black populations (77% by age 20, ~100% over age 50); melanocytic activation vs hyperplasia pathophysiology; average ~2-year diagnostic delay for subungual melanoma
  2. 2.Ricardo JW, Bellet JS, Jellinek N, Lee D, Miller CJ, Piraccini BM, Richert B, Rubin AI, Lipner SR (2025). Evaluation and diagnosis of longitudinal melanonychia: A clinical review by a nail expert group. Journal of the American Academy of Dermatology. doi:10.1016/j.jaad.2025.02.075 ✓Classification of causes of longitudinal melanonychia; melanocytic activation vs hyperplasia framework; pediatric conservative management rationale; monitoring vs biopsy approach
  3. 3.Moulonguet I, Caucanas M, Goettmann S (2026). Longitudinal Melanonychia in Children: Clinical and Histopathologic Features and Management with Literature Update. Dermatopathology (Basel). doi:10.3390/dermatopathology13010013 ✓Nail matrix nevus as the predominant cause in children; conservative watch-and-wait management in pediatric longitudinal melanonychia; risk of nail dystrophy from biopsy in young children
  4. 4.Senet P, Pindard J, Lebbe C, Sergent-Alaoui A (2010). Melanonychia and skin hyperpigmentation with hydroxyurea therapy. Annales de Dermatologie et de Venereologie (PMC). linkHydroxyurea, cyclophosphamide, doxorubicin, minocycline, and zidovudine as documented drug causes of melanonychia; nail changes reversible after drug withdrawal
  5. 5.Patel GA, Ragi G, Krysicki J, Schwartz RA (2008). Subungual melanoma: a deceptive disorder. Acta Dermatovenerol Croat. PMID 19111151 ✓Subungual melanoma 0.7-3.5% of melanomas overall; 75% of melanomas in African populations; 25% in Hong Kong Chinese; 10% in Japanese; ~1/3 amelanotic; Hutchinson sign description; late diagnosis and poor prognosis
  6. 6.Wang Y, Zhao Y, Ma S (2016). Racial differences in six major subtypes of melanoma: descriptive epidemiology. BMC Cancer. doi:10.1186/s12885-016-2747-6 ✓Acral lentiginous melanoma represents up to 75% of melanomas in non-Caucasian patients; racial disparities in melanoma subtype distribution
  7. 7.Ohn J, Jo G, Cho Y, Sheu SL, Cho KH, Mun JH (2018). Assessment of a Predictive Scoring Model for Dermoscopy of Subungual Melanoma In Situ. JAMA Dermatology. doi:10.1001/jamadermatol.2018.1372 ✓Hutchinson sign OR 18.18 for subungual melanoma in situ; dermoscopic scoring model AUC 0.91 with 89% sensitivity and 62% specificity; six dermoscopic features associated with malignancy
  8. 8.Oh SJ, Lee J, Park S, Park JH, Lee D (2022). Hutchinson Sign: Biopsy May Assist in Diagnosis of Subungual Melanoma in Situ. Dermatologic Surgery. PMID 34608084 ✓Hutchinson sign not pathognomonic for melanoma; pseudo-Hutchinson sign in benign conditions; biopsy of Hutchinson sign area as a supplementary diagnostic technique
  9. 9.Ko D, Oromendia C, Scher R, Lipner SR (2019). Retrospective single-center study evaluating clinical and dermoscopic features of longitudinal melanonychia, ABCDEF criteria, and risk of malignancy. Journal of the American Academy of Dermatology. doi:10.1016/j.jaad.2018.08.033 ✓ABCDEF criteria description; band width >40% of nail plate as biopsy threshold; ABCDEF criteria count alone did not distinguish melanoma from benign lesions in 84-case cohort

9 sources, numbered by first appearance. General health information, not medical advice — synthetic demonstration content.