Background: Atypical nevi (AN) are regarded as a major risk factor for occurrence of malignant melanoma. Few studies report an increase in relative risk for melanoma in patients with nonfamilial AN. Objective: We measured the risk of melanoma in 195 patients with nonfamilial AN. Methods: In a retrospective follow-up study the number of newly diagnosed melanomas during follow-up were inventoried. Included patients were invited for skin screening and nevus count. Results: On the basis of our calculations, the relative risk for invasive melanoma in patients with nonfamilial AN, including those patients with a history of melanoma, is 41. For individuals with AN without a previously diagnosed melanoma, the relative risk was 37. Multivariate analysis did not reveal a specific risk factor for the occurrence of melanoma (including number of AN) because of small numbers. Invasive melanomas occurred in persons having 1, 5, 12, and 172 AN, respectively. Conclusion: An increased risk for invasive cutaneous malignant melanoma in patients with nonfamilial AN was established.(J Am Acad Dermatol 1999;40:686-93.)