We evaluated the risk of developing melanoma over time in members of 23 melanoma-prone families. All 23 families had dysplastic nevi as well as melanoma. Forty-seven melanomas occurred prospectively, all in family members with dysplastic nevi. The prospective melanomas were markedly thinner than the melanomas diagnosed prior to or at the time of the subject's entry into the study. The cumulative risk of melanoma by age 50 years among people with dysplastic nevi was 48.9% +/- 4.2%. Overall, the relative risk of a prospective melanoma among family members with previous melanoma was 229 (95% confidence interval 110-422). The risk varied by time interval and was 362 in the first 5 years, decreasing to 120 after 5 years. The risk of developing melanoma was 85 times increased (95% confidence interval 41-156) in family members with dysplastic nevi and also declined over time in this group. There was no significant excess of cancers other than melanoma. Close surveillance of these high-risk families has led to diagnosis of melanoma at an earlier developmental stage, which should result in a decrease in mortality over time.