Estimates have been made of the proportion of cutaneous malignant melanomas caused by sun exposure by comparing the observed incidence of melanoma with estimates of the incidence in the absence of sun exposure. The estimated proportions varied from 0.97 in males and 0.96 in females in Queensland, Australia, when the Incidence on the whole body was compared with that on unexposed sites, to 0.68 when Incidence In people born in Australia was compared with that in migrants to Australia from areas of lower sun exposure. A comparison of US Whites and US Blacks, in which the Incidence In Blacks was taken as the Incidence In unexposed Whites, gave estimates of 0.96 in males and 0.92 In females. It was estimated that some 59 000 (65%) of about 92000 melanomas that occurred worldwide in 1985 were caused by sun exposure. This is probably a minimum estimate. That 20% of the world's melanomas are estimated to occur in Black African and Asian populations and are of unknown cause would justify studies of the causes of melanoma In these populations.