The causes of renal cell cancer (RCC) are poorly understood. Besides smoking, obesity remains the only risk factor that is fairly well established. The association between obesity and RCC appears stronger and more consistent in women than in men. We investigated the question of whether this apparent sex difference could be explained by repeated weight changes (weight cycling), less physical exercise, or pharmacological treatment of obesity in women. Structured face-to-face interviews were carried out with 379 (70% of all eligible) incident cases of RCC and 353 (72% of eligible) controls. The relationships between RCC and adult height, weight, and body mass index (BMI), defined as weight/height, were analyzed. Odds ratios (ORs) were estimated through logistic regression. No association was found between adult height and RCC. In men, weight and BMI appeared at most to be weakly related to risk of RCC. In women, higher adult weight and BMI (usual, highest, and lowest) and also high BMI at ages 30, 40, and 50 years were consistently associated with a significantly increased risk of RCC. Women with an usual adult BMI in the top 5% had a nearly 3-fold increased risk of RCC [OR, 2.67; 95% confidence interval (CI), 1.02-7.01]. Compared with individuals with no weight-loss periods, 2 or more such periods implied an OR of 0.96 (95% CI, 0.32-2.90) in men and 3.87 (95% CI, 1.20-12.45) in women. Physical activity at work reduced the risk of RCC in men but not women. Regular use of diet pills containing amphetamine was associated with an increased risk of RCC (OR, 4.06; 95% CI, 1.35-12.22).(ABSTRACT TRUNCATED AT 250 WORDS)