Evidence associating malignant melanoma with semiquantitative and questionnaire indicators of past sunlight exposure is presented from a case-control study of 511 patients and 511 matched control subjects in Western Australia. That melanoma is related to sun exposure was supported by associations with actinic skin damage graded by cutaneous microtopography, history of nonmelanotic skin cancer, duration of residence of migrants to Australia, and mean annual hours of bright sunshine received at locations where the subjects had resided. Separate analyses of histogenetic subtypes revealed that Hutchinson's melanotic freckle melanoma had the strongest associations with indicators of sunlight exposure. For superficial spreading melanoma, a specific relationship was observed with age at arrival as against duration of residence in Australia. Migrants arriving before age 10 years appeared to have a risk similar to that of native-born Australians, whereas the estimated incidence in those arriving after age 15 years was around one-quarter of the native-born rate, with arrival at later ages giving no additional advantage. Control subjects arriving in Australia before age 10 years had an increased number of nevi on their arms, suggesting that sun exposure in early life may be a factor in nevus production and, therefore, a determinant of later potential to develop superficial spreading melanoma.