Objective: To report on the presentation, histopathologic findings, and clinical outcomes for a case series of MIS of the nail apparatus because melanoma in situ (MIS) of the nail unit has not been well characterized in the literature. Setting: A division of a tertiary academic center specializing in micrographic excision of cutaneous neoplasms. Design: Surgical records were searched for cases of MIS of the nail unit for the period of January 1, 1997, to December 31, 2002. The patient demographics and disease presentation, treatment, and clinical course were reviewed. Results: Seven cases of MIS of the nail unit in white patients were identified. Longitudinal melanonychia was present in all cases, but dyspigmentation of the proximal nail fold and onychodystrophy were uncommon. Histopathologic analysis revealed poorly circumscribed proliferations of single cells over nests with variable pagetoid spread. Atypia was variable. Mitotic activity was low. All cases were treated with micrographic surgery. Amputation was avoided in 3 cases and was limited to partial distal interphalangeal amputation in the remainder. Six cases did not recur locally after initial surgical intervention. With an average of 24 months of follow-up, all patients were free of disease. Conclusions: Longitudinal melanonychia in a white patient mandates consideration of MIS of the nail unit. Given the nondescript clinical presentation, the threshold for biopsy should be low. The histopathologic findings appear similar to those of MIS in other areas, with asymmetry and poor circumscription predominating. With additional study and further acceptance, micrographically controlled excision has the potential to minimize morbidity. Further investigation is warranted. Objective: To report on the presentation, histopathologic findings, and clinical outcomes for a case series of MIS of the nail apparatus because melanoma in situ (MIS) of the nail unit has not been well characterized in the literature. tertiary academic center specializing in micrographic excision of cutaneous neoplasms. Design: Surgical records were searched for cases of MIS of the nail unit for the period of January 1, 1997, to December 31, 2002. The patient demographics and disease presentation, treatment, and Clinical course were reviewed. Results: Seven cases of MIS of the nail unit in white patients were identified. Longitudinal melanonychia was present in all cases, but dyspigmentation of the proximal nail fold and onychodystrophy were uncommon. Histopathologic analysis revealed poorly circumscribed proliferations of single cells over nests with variable pagetoid spread. Atypia was variable. Mitotic activity was low. All cases were treated with micrograp hic surgery. Amputation was avoided in 3 cases and was limited to partial distal interphalangeal amputation in the remainder. Six cases did not recur locally after initial surgical intervention. With an average of 24 months of follow-up, all patients were free of disease. Conclusions: Longitudinal melanonychia in a white patient mandates consideration of MIS of the nail unit. Given the nondescript clinical presentation, the threshold for biopsy should be low. The histopathologic findings appear to be similar in MIS in other areas, with asymmetry and poor circumscription predominating With additional study and further acceptance, micrographically controlled excision has the potential to minimize morbidity. Further investigation is warranted.