Sunlight is the major environmental risk factor for melanoma. Descriptive studies have shown latitudinal variation in population incidence and mortality rates [D. C. Whiteman and A. C. Green, Int. J. Dermatol., 38: 481-489, 1999, and B. K. Armstrong, Australian J. Dermatol., 38 (Suppl. 1): 51-56, 1997]. In analytic studies, individual exposure has been particularly difficult to quantify. Lifetime residential history was coupled with levels of midrange UV radiation (UVB flux) to provide a measure of individual exposure to sunlight thought to be less subject to misclassification and recall bias. Data were analyzed from 718 non-Hispanic white patients with invasive cutaneous melanoma from melanoma clinics in Philadelphia and San Francisco. Matched controls were 945 patients from outpatient clinics with similar catchment areas. The association of melanoma risk and history of UVB flux along with the usual outdoor exposure risk factors were studied. A 10% increase in the average annual UVB flux was associated with a 19% [95% confidence interval (CI), 5-35%] increase in individual odds for melanoma for men and 16% (95% CI, 2-32%) for women. In men, a 10% increase in hours outdoors was associated with a 2.8% (95% CI, 1.2-4.5%) increase in odds. Even in women who could develop a deep tan, a 10% increase in hours outdoors was associated with a 5.8% increase in odds (95% CI, 1.4-10.4%). The association between melanoma risk and average annual UVB flux was strong and consistent for men and for women. The association with total adult hours outdoors was notable for men of all skin types and women who develop a suntan.