Longitudinal melanonychia describes a longitudinal band of pigment that extends from the nail matrix to the free edge of the nail. Melanonychia means "black nail." There are many causes of nail pigmentation (see Differential Diagnosis section) including skin diseases, systemic diseases, infections, trauma, medications, and pigmented lesions. While longitudinal melanonychia can have a benign etiology, it is critical to rule out acral lentiginous melanoma as a cause.

Melanocytes normally reside in the nail matrix and nail bed, though they are typically inactive in individuals of Northern European descent. In the nail matrix, if melanin production by melanocytes is greater than degradation by keratinocytes, the result is a longitudinal band. This phenomenon can be caused by melanocyte activation, melanocytic hyperplasia, lentigo simplex, melanocytic nevus, atypical melanocytic proliferations, and acral lentiginous melanoma.

Pediatric nail melanoma is extremely rare, particularly in individuals with Fitzpatrick phototypes I and II, but carries significant morbidity and mortality. The fingernails are more commonly involved than toenails. Evaluation of longitudinal melanonychia is more difficult in children than adults due to lack of clinical and histopathological guidelines. According to recent cohort studies from Asia, the majority of pediatric cases undergo regression or stagnation, and no incidence of melanoma was found in biopsied cases.