While the association between large congenital nevocellular nevi and malignant melanoma has been established beyond reasonable doubt, its magnitude is the subject of wide controversy. In this study an attempt is made to estimate the risk of malignant transformation during the first 15 years of life in a sample of 39 congenital nevocellular nevi occupying a mean of 17 percent of the total body surface area collected from plastic surgical centers in England and Wales. During the period of observation, which totaled 337 years, two patients developed and died of metastatic malignant melanoma. A melanoma risk of 8.52 percent during the first 15 years of life is calculated. Although a high risk of melanoma must weigh heavily in favor of early prophylactic excision, the management of large congenital nevocellular nevi is considered to be too complicated to permit the invariable adoption of a wholesale excisional policy.