151 patients with benign giant pigmented nevi registered in the Danish Health system during the 60-year period 1915-75 were retrieved via the national register. A questionnaire was sent to all surviving patients asking for information about their health and especially whether they had had any treatment or had observed any changes in the nevus. All of the patients replied to the questionnaire. No patients had been cured from malignancy or were alive with known malignancy. Three patients had died from malignant melanoma during the period of observation. These case histories are reported. It is calculated that 4.6% of the patients with congenital giant nevi should be expected to develop malignant melanoma provided the incidence is the same in all age groups. Some uncertainty remains on account of the limited number of cases and an unsettled question about a higher incidence in childhood.