Background: Because most dermatologists do not regularly perform biopsies of longitudinal melanonychia, even when the pigmentation presents as a single band, the true prevalence of nail matrix nevi is unknown. Objective: Our purpose was to determine the prevalence of nail matrix nevi in white patients with longitudinal melanonychia involving a single digit and to determine whether longitudinal melanonychia caused by a nail matrix nevus can be clinically distinguished from longitudinal melanonychia from other causes. Methods: From January 1989 to December 1994 we performed a nail biopsy on 100 of 128 consecutive white patients who had a single band of 'idiopathic' longitudinal melanonychia. Results: A nail matrix nevus was detected in 22 patients. A junctional nevus was found in 19 specimens and a compound nevus in three specimens. Conclusion: Nail matrix nevi in Caucasian patients are uncommon but not exceptional. The number of nevi presenting with longitudinal melanonychia exceeded that of melanoma. The diagnosis of nail matrix nevi is impossible clinically and always requires histopathologic study. The pathologic features of nail matrix nevi are similar to those of skin nevi except for their architectural pattern, which reflects the peculiar anatomy of the nail unit.
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