The purpose of this study was to assess the sensitivity of clinical diagnosis of cutaneous malignant melanoma and to evaluate histologic characteristics of lesions not clinically diagnosed as such. Of 1,784 cases of histologically proven cutaneous malignant melanoma submitted routinely to a university dermatopathology laboratory between 1985 and 1990, 583 (33%) were not clinically suspected. The overall sensitivity in clinical diagnosis was 67%. Histologic features evaluated included presence of melanin, pagetoid spread of melanocytes, degree of inflammation, regression, presence and degree of sun damage as evidenced by solar elastosis, presence of melanin in the cornified layer, and coexisting nevus cells. Melanomas clinically thought to be nevi had less solar elastosis and most frequently had associated nevus cells. Those thought to be basal cell carcinomas had less melanin in lesions and less melanin in the cornified layer, and most often had foci of regression. Lesions thought to be keratoses showed melanin in the cornified layer 70% of the time, more often than any other type of lesion. Melanoma may be unsuspected clinically in a significant number of cases and can be mistaken for less serious cutaneous neoplasms. Histologic features of these lesions correlated well with original clinical diagnoses.