The study presented here is based on the clinicopathological and electron microscopical analysis of 39 patients with Becker's nevus (BN). The clinical picture is variable and we therefore suggest the following three types be distinguished: (1) the melanotic type, (2) the hypertrichotic type, and (3) the mixed type. In BN the sensitivity reaction was often diminished. Among the skin eruptions other than BN, we would like to stress that 12% of our patients showed a malignant melanoma. Histopathology concerns both epidermal and dermal structures. The electron microscopic findings corresponded well with those by light microscopy. As far as we know, the changes in collagen fibrils are demonstrated for the first time in this study.