Subungual melanomas are one of the most common types of malignant melanoma among the Japanese population. Although most pigmented nail streaks are benign and remain unchanged in their color and shape for a long time, rarely are they precursor lesions of subungual melanomas, i.e., a rapid growing pigmented nail streak resulting in diffuse melanosis of the nail is thought to be an early stage of subungual melanoma in situ. We found four patients with these changes; three of these patients were children. The lesions occurred on the right index finger, right thumb, left middle finger, and right great toe, respectively. A slightly haphazard combination of colors ranging from dark brown to black, the important characteristic of subungual melanoma in situ, was observed in two cases. In the remaining two cases, although the haphazard combination of colors was not distinctive, many fine, dark, longitudinal lines were seen within diffuse, light-brownish pigmentation. Serial histologic examination of the excised tissue specimens showed great proliferation of vacuolated melanocytes with variable nuclear atypicality along the entire basal layer in all cases. These histologic changes were compatible with those of atypical melanocytic hyperplasia or intraepidermal melanoma (in situ melanoma), which is an early lesion of subungual melanoma. An adult case is thought to be a definite example of a subungual melanoma in situ. We also made the diagnosis of melanoma in situ in the remaining three cases of children with rapidly growing pigmented nail streaks because their histopathologic features were distinguishable from those of the adult case. However, there remains some hesitation about this because invasive subungual melanoma is rare in children. Cancer 64:2191-2197, 1989. ACIAL DIFFERENCES are pronounced in the inci-R dence and predilection of melanoma types. Malignant melanoma occurs much less frequently among Jap-anese individuals than among white individuals, and melanoma occurring in the acral regions including the subungual position is the most common type of mela-noma among the Japanese population.' Reflecting its predilection, there have been several case reports of the acral type of melanoma in situ in the Japanese litera-t ~ r e , ~-~ including one report of subungual melanoma in sir^.^ However, only one case report has been published about palmar melanoma in situ in a white woman.6 Currently , however, in all of the instances a sharp demarcation cannot be made between atypical melanocytic hyperplasia (melanocytic dysplasia) and in situ