In a population-based case-control study including 449 directly interviewed cases and 707 controls, we assessed the risk of renal cell cancer associated with height, weight, body mass index (BMI), and frequency of weight changes. Odds ratios and 95% confidence intervals were estimated by using logistic regression models. Among women, risk increased with increasing usual BMI (P for trend < 0.001). A nearly 4-fold risk was found among the 10% of women with the highest usual BMI (odds ratio = 3.8; confidence interval = 1.7-8.4). Among men, no clear trend was observed with usual weight or BMI, although the highest risk (30-50%) generally was seen among those in the upper deciles of weight or BMI. There was no clear indication that excess BMI early or late in life disproportionately affected risks. Risk also was not related to patterns of weight fluctuations or use of diet pills. Our study supports previous observations linking renal cell cancer risk to increased BMI among women and suggests a weaker association in men. Given the increasing prevalence of obesity and the rising incidence of renal cell cancer in the United States, additional studies are needed to disentangle the effects of BMI from various correlates and to identify the mechanisms by which obesity affects risk.