Constitutional factors and sun exposure were examined among 256 cases of melanoma and 273 controls in three counties of western Washington State. Cases were individuals diagnosed with melanoma at ages 25-65 years during 1984-1987 who were identified from a Seattle-Puget Sound cancer registry. Population controls were identified through random digit dialing and were randomly selected, stratified by age, sex, and county. Participants completed a telephone interview with questions on demographic, sun exposure, and constitutional factors. Risk factors for melanoma were examined through the use of logistic regression, controlling for age, sex, and educational level, and data on each exposure variable were tested for the significance of trends across levels of exposure. The constitutional factor most strongly associated with melanoma risk was sun sensitivity, measured as either reaction to chronic sun exposure (for no tan vs. deep tan, odds ratio (OR) = 9.0, 95% confidence interval (CI) 3.8-21.1; p < 0.001 for trend across four categories) or reaction to acute sun exposure (for severe burn vs. tan, OR = 5.7, 95% CI 2.6-12.6; p < 0.001 for trend across four categories). The number of raised nevi counted on both arms by subjects was also associated with increasing risk (for > or = 10 nevi vs. none, OR = 5.7, 95% CI 2.2-14.6; p for trend < 0.001). Sun exposure in adulthood and occupational sun exposure were not related to melanoma risk. The effect of sun exposure in childhood on melanoma risk was modified by tanning ability. Poor tanners showed no effect of sun exposure at ages 2-10 years or ages 11-20 years. In contrast, people who reported a deep or moderate tan in reaction to chronic sun exposure appeared to be protected from melanoma with increasing sun exposure at ages 2-10 years (for upper third vs. lower third, OR = 0.3, 95% CI 0.2-0.6; p for trend < 0.001), with identical results for exposure at ages 11-20 years. The finding that childhood sun exposure is protective only among those able to tan supports the hypothesis that developing a tan during childhood may reduce the risk of melanoma by offering protection from the effects of sunlight exposure.