In a retrospective follow-up study, 36 renal transplant recipients with, and 101 without, skin cancer, who had received their first transplant before January 1981 and who were still alive with a functioning graft on 1 August 1989, were assessed to determine the risk of non-melanoma skin cancer in relation to exposure to sunlight during childhood and adolescence. The contribution of the number of keratotic skin lesions to the skin cancer risk was also assessed. The estimated relative risks (odds ratios) of skin cancer in relation to exposure to sunlight and the presence of keratotic skin lesions were calculated by maximum likelihood estimation in a logistic model. The majority of skin cancers and keratotic skin lesions were confined to sun-exposed skin. After adjustment for possible confounding variables, the odds ratios of skin cancer for moderate and high cumulative life-time exposure to sunlight, respectively, compared with low exposure, were 2.4 (95% confidence interval [CI] 0.64-9.3) and 47.6 (95% CI 5.4-418). Exposure to sunlight before the age of 30 contributed more to the risk of developing skin cancer later in life than exposure after the age of 30. No association was found between cumulative life-time exposure to sunlight and the number of keratotic skin lesions. Nevertheless, these lesions behaved as a strong independent risk factor in the development of skin cancer. The adjusted odds ratio of skin cancer for 50-99 lesions compared with < 50 lesions was 4.5 (95% CI 1.1-18.2); the adjusted odds ratio for > or = 100 lesions compared with < 50 lesions was 20.8 (95% CI 5.3-81.7).(ABSTRACT TRUNCATED AT 250 WORDS)