A case control study of risk factors for malignant melanoma and nonmelanoma skin lesions was conducted. The cohort consisted of 289 patients with histologically confirmed melanoma diagnosed between 1979 and 1982 and 75 patients with nonmelanoma skin lesions, such as solar keratosis, basal cell carcinoma, and squamous cell carcinoma, known to be associated with sun exposure. The comparisons consisted of 527 persons with nonmalignant skin disorders. The subjects were interviewed to obtain information on sun exposure, history of sunburns, and history of nonmelanoma skin cancer or solar keratosis. Odds ratios (ORs) for melanoma were determined by polychotomous logistic regression techniques. The risk of melanoma was significantly elevated for subjects having little or no ability to tan, ORs 2.0 and 1.7, respectively. An average ability to tan was associated with a decreased risk, OR 0.7. The risk of nonmelanoma was significantly elevated among subjects with little or no ability to tan, ORs 2.4 and 1.8, respectively. A history of freckling was associated with an increased risk for melanoma but not nonmelanoma, OR 1.9. The melanoma risk was increased for persons with more than 25 moles on their body. Having blue eyes was associated with significantly increased risks for melanoma and nonmelanoma, ORs 2.6 and 1.9, respectively, relative to those with brown eyes. The melanoma and nonmelanoma risks were significantly increased for northern European paternal ethnicity, ORs 1.6 to 3.2. Mixed indoor and outdoor recreational exposure was associated with a 50% increased risk for nonmelanoma, but a 25% decreased risk for melanoma. Subjects with histories of large lifetime sun exposure had significantly elevated risks for melanoma and nonmelanoma, ORs 1.7 and 2.0, respectively. Moderate sun exposure was not associated with an increased risk for either condition. Severe sunburns were associated with elevated risks for melanoma and nonmelanoma, ORs 1.6 and 1.8, respectively. A history of nonmelanoma skin cancer or sun keratosis was associated with increased risks for melanoma and nonmelanoma, ORs 7.3 and 20.2, respectively.