Background: The risk of the occurrence of malignant melanoma (MM) in medium-sized (1.5 to 19.9 cm in diameter) congenital melanocytic nevi (CMN) is the subject of controversy. Universally accepted recommendations regarding the management of such lesions have not been made. Objective: Our purpose was to assess the risk of MM arising in medium-sized CMN. Methods: The study included 230 medium-sized CMN in 227 patients, first seen in a private dermatology practice from 1955 to 1996, who were followed up for MM arising within their CMNs. Criteria for entry into the study included (1) a clinically diagnosed medium-sized CMN, (2) minimum follow-up period of 1 year, and (3) a photograph of the lesion in the patient’s medical record. Results: No MM occurred in a medium-sized CMN during an average follow-up of 6.7 years (median, 5.8 years) to an average age of 25.5 years (median, 19.1 years). Conclusion: The results of this short-term follow-up study do not support the view that there is a clinically significantly increased risk for MM arising in banal-appearingmedium-sized CMN or that prophylactic excision of all such lesions is mandatory. Life-long medical observation seems a reasonable alternative for many medium-sized CMN. (J Am Acad Dermatol 1998;39:428-33.)