The proposed case-control study will determine risk factors for dysplastic nevi, the most important potentially preventable precursor (and risk factor) for melanoma. Melanoma is projected to be more common than colon cancer among cohorts born after 1950, and continues to increase in both incidence and mortality rates. The identification of risk factors for dysplastic nevi will not only aid in efforts to prevent melanoma, but may allow for prevention of dysplastic nevi and the associated need for periodic dermatologic examinations with the attendant anxieties and multiple surgical excisions. Furthermore, it could allow us to better understand the pathogenesis of melanoma by determining which risk factors act at each stage in its evolution. The hypothesis that sunlight is a cause of dysplastic nevi will be examined. Sunlight is a complex variable which includes traits indicative of sensitivity to the sun as well as measures of actual exposure, as indicated by location of residence and vacations, occupational and recreational activities and type of clothing worn. Certain items, such as the number of blistering sunburns received, are a function of both sun sensitivity and exposure. If sunlight is associated with dysplastic nevi, the timing of the exposure will be examined to investigate the possibility of a critical period or a latent period. The anatomic site would also be examined to distinguish between local and systemic effects of sunlight. Family history of melanoma and dysplastic nevi, oral contraceptive use, pregnancy history, the presence of nevi at birth and alcohol consumption will also be evaluated. Risk factors will be determined by telephone interview of histologically confirmed cases of dysplastic nevus and age and sex matched controls. To minimize possible referral bias, controls will be selected from the same Health Maintenance Organization as the cases. One control group also will be matched to cases by visit to primary care provider in order to further minimize possible bias.