The dysplastic nevus (DN) is the most important risk factor and precursor for malignant melanoma. The authors compared the responses of 132 consecutive cases of DN, 186 consecutive cases of cutaneous melanoma, and 239 controls attending the same subspecialty clinic to questions regarding sun sensitivity, sun exposure, and other possible risk factors. Dysplastic nevus cases were younger than controls and were of a higher social class, as indicated by more years of formal education. Sun sensitivity (assessed by reported depth of tan after multiple exposures) was associated with both DN risk and melanoma risk after controlling for age and education in logistic regression analysis (P = 0.009 and 0.03, respectively), but for DN risk this association was nonlinear: the relative risks (versus deep tan) were 2.3 for average tanners, 2.8 for light tanners, and 1.6 for those who reported practically no tan. Sun exposure measures were not associated with DN risk after controlling for age and education, whether or not depth of tan was controlled in the analysis. These observations suggest a role for either sunlight or a trait linked with sun sensitivity in the development of dysplastic nevi.