To examine the association between body mass index and renal cell carcinoma risk, we analyzed data from a case-control study of members of a health maintenance organization in western Washington State. We identified cases diagnosed between 1980 and 1995 through a population-based cancer registry. We selected controls from membership files. We collected adult weight and height from medical records. Increased body mass index was associated with increases in risk for both men and women (for the top quartile relative to the bottom quartile of maximum body mass index: in women, OR = 3.3, 95% CI = 1.2-8.7; in men, OR = 2.3, 95% CI = 1.2-4.5).