Catching sight of a narrow brown line running the length of a fingernail tends to produce one of two reactions. Either it gets waved away as a knock you cannot quite remember, or it becomes the reason you are still awake at two in the morning reading about melanoma.
Neither response is much use, because the honest answer sits in between. A pigmented band in a nail is common and usually benign, and it is also the single presentation through which nail unit melanoma announces itself. Telling those apart is a clinical task, not a task for a phone torch, which is why skin cancer screening exists as a service in the first place.
What the pigment in a nail is
The medical term is longitudinal melanonychia, meaning a brown to black band running from the cuticle towards the free edge of one or more nails1. The pigment is melanin, produced by melanocytes sitting in the nail matrix, the growth zone tucked beneath the cuticle where the nail plate is formed.
There are two broad mechanisms behind it. In melanocytic activation, existing pigment cells simply start producing more melanin, which is what happens after friction, trauma, certain medicines, inflammatory nail disease, or as a normal constitutional feature. In melanocytic hyperplasia, the number of pigment cells increases, which covers benign naevi of the nail matrix as well as melanoma1.
The band itself cannot tell you which mechanism is at work. That is the entire difficulty.
Why most dark streaks turn out to be harmless
Benign melanonychia is encountered often in routine practice, and it behaves differently depending on skin phototype. In a ten-year cohort of 248 benign cases, melanocytic activation was the commonest cause, and patients with darker skin had wider bands, more change over time, and consequently received more biopsies than patients with lighter skin2. For a Singaporean readership, that cuts both ways. Constitutional pigment bands are more likely to be part of your normal, and the features that would otherwise raise suspicion are also more likely to appear without disease behind them. It makes self-assessment less reliable, not more.
The features that change the conversation
Dermatologists work from a recognised set of warning features, originally set out as an ABCDEF rule for the clinical detection of subungual melanoma, covering age and ethnic background, the breadth, border and colour of the band, change in the band or failure to respond to treatment, which digit is involved, extension of pigment onto the surrounding skin, and family or personal history of melanoma3. The features that most often prompt a biopsy are:
- A band that is widening, or occupies a large proportion of the nail plate.
- Blurred or fading borders instead of crisp parallel edges.
- More than one colour within the band, or irregular shades of brown, grey and black.
- Pigment spreading onto the cuticle or the skin at the nail fold, known as Hutchinson’s sign.
- A single affected nail in an adult, particularly a thumb, index finger or great toe.
- Splitting, thinning or distortion of the nail plate over the pigmented area.
This list is a prompt to get assessed and not a way to clear yourself. A dermoscopic scoring model developed in 45 Korean patients reached 89% sensitivity and 62% specificity for subungual melanoma in situ at its chosen cut-off4, which is useful in trained hands and still leaves cases that only histology can settle.
Width deserves particular attention over time. Pigmentation measuring 3 mm or more across was one of the five features predicting melanoma in situ in that model, alongside asymmetry, fading borders, multiple colours and pigment spreading onto surrounding skin4. A photograph taken today beside a ruler is therefore far more useful at a review appointment than a description from memory.
Why the local picture differs
Melanoma subtype distribution is not the same everywhere. A twenty-year review of 62 melanomas at a Singapore dermatology centre found acral lentiginous melanoma in 35.5% of cases, with 45.2% arising at palmoplantar or subungual sites, and Asian patients presented with significantly thicker tumours than Caucasian patients, at a median of 2.6 mm against 0.9 mm5. That is a single-centre series rather than a national rate, so it should not be read as a personal risk figure, but the pattern it describes deserves knowing: in this part of the world melanoma is comparatively more likely to appear on a palm, a sole or under a nail, which are exactly the sites people forget to check. Knowing how to check for skin cancer elsewhere on the body is only half the habit if hands and feet are skipped, and the HealthHub guide to recognising skin cancer spots is a sound starting point for the rest.
How it is assessed
Assessment begins with history and a careful look at every nail, since a pattern across several nails points away from malignancy. Onychoscopy, which is dermoscopy applied to the nail, allows the band’s width, colour and border to be examined properly, and nail clipping histopathology can contribute further information. Where suspicion remains, excisional biopsy of the nail matrix is the definitive step and the accepted standard for ruling out malignancy1.
A matrix biopsy is a small procedure performed under local anaesthetic which is typically a ring block, and it carries a real possibility of a permanent ridge or split in the nail afterwards. That trade-off is discussed before anything is done, and it is the reason a dermatologist will not biopsy every pigmented band that walks through the door.
Children are a different case
Pigment bands in childhood behave more benignly. Among 174 nails from 172 Chinese patients, nail matrix naevus accounted for 56.1% of paediatric cases against 34.3% of adult cases, and while six adults in that series had subungual melanoma, no child did6. Bands were wider in adults, with width correlating with how long the band had been present. Paediatric melanonychia still deserves review, and parents are usually reassured rather than alarmed by it.
Conclusion
A pigmented nail band is a question that can be answered, and answering it early is considerably easier than answering it late. Most people who have theirs examined are told it is benign and given a plan for keeping an eye on it, which is a better outcome than months of quiet worry.
At Angeline Yong Dermatology, Dr Angeline Yong evaluates nail pigmentation, examines the surrounding nail unit, and advises on whether a biopsy is warranted, with diagnostic and surgical options available at the clinic. Book a consultation if a line on your nail has appeared, widened or changed.
References
Ricardo, J. W., Bellet, J. S., Jellinek, N., Lee, D., Miller, C. J., Piraccini, B. M., Richert, B., Rubin, A. I., & Lipner, S. R. (2025). Evaluation and diagnosis of longitudinal melanonychia: A clinical review by a nail expert group. Journal of the American Academy of Dermatology, 93(1), 176–187. https://doi.org/10.1016/j.jaad.2025.02.075
Lee, D. K., Chang, M. J., Desai, A. D., & Lipner, S. R. (2022). Clinical and dermoscopic findings of benign longitudinal melanonychia due to melanocytic activation differ by skin type and predict likelihood of nail matrix biopsy. Journal of the American Academy of Dermatology, 87(4), 792–799. https://doi.org/10.1016/j.jaad.2022.06.1165
Levit, E. K., Kagen, M. H., Scher, R. K., Grossman, M., & Altman, E. (2000). The ABC rule for clinical detection of subungual melanoma. Journal of the American Academy of Dermatology, 42(2), 269–274. https://doi.org/10.1016/S0190-9622(00)90137-3
Ohn, J., Jo, G., Cho, Y., Sheu, S. L., Cho, K. H., & Mun, J.-H. (2018). Assessment of a predictive scoring model for dermoscopy of subungual melanoma in situ. JAMA Dermatology, 154(8), 890–896.
Yeo, P. M., Lim, Z. V., Tan, W. D. V., Zhao, X., Chia, H. Y., Tan, S. H., Teo, M. C. C., & Tan, M. W. P. (2021). Melanoma in Singapore: A 20-year review of disease and treatment outcomes. Annals of the Academy of Medicine, Singapore, 50(6), 456–466. https://doi.org/10.47102/annals-acadmedsg.2020535
Lyu, A., Hou, Y., & Wang, Q. (2023). Retrospective analysis of longitudinal melanonychia: A Chinese experience. Frontiers in Pediatrics, 10, 1065758. https://doi.org/10.3389/fped.2022.1065758