Prospective follow-up for malignant melanoma in patients with atypical-mole (dysplastic-nevus) syndrome. A total of 357 white patients who had melanocytic nevi that fulfilled the clinical criteria for the "classic" atypical-mole (dysplastic-nevus) syndrome (100 or more melanocytic nevi; one or more melanocytic nevi 8 mm or larger in diameter; and, one or more melanocytic nevi with atypical features) were followed for the development of cutaneous malignant melanomas. Seventeen patients (4.8%) developed malignant melanomas during an average follow-up period of 49 months. One patient developed two malignant melanomas. Eight of the malignant melanomas detected were in situ and ten were invasive melanomas (less than 0.86 mm in Breslow thickness), implying an excellent prognosis. The number of malignant melanomas detected in these patients exceeded significantly the number expected to occur in age- and sex-matched white controls. All groups were shown to have an increased risk for the development of malignant melanomas. Total-body photographs were helpful in detecting changes in size, shape, and color that led to the diagnosis of malignant melanoma. These data support the concept that patients with this readily regionalized clinical presentation of classic atypical-mole syndrome are at an increased risk for malignant melanomas and, therefore, should be examined regularly. 