BACKGROUND: Although interest in the relationship between obesity and kidney disease is increasing, few epidemiologic studies have examined whether excess weight is an independent risk factor for end-stage renal disease (ESRD). OBJECTIVE: To determine the association between increased body mass index (BMI) and risk for ESRD. DESIGN: Historical (nonconcurrent) cohort study. SETTING: A large integrated health care delivery system in northern California. PARTICIPANTS: 320,252 adult members of Kaiser Permanente who volunteered for screening health checkups between 1964 and 1985 and who had height and weight measured. MEASUREMENTS: The authors ascertained ESRD cases by matching data with the U.S. Renal Data System registry through 2000. RESULTS: A total of 1471 cases of ESRD occurred during 8,347,955 person-years of follow-up. Higher BMI was a risk factor for ESRD in multivariable models that adjusted for age, sex, race, education level, smoking status, history of myocardial infarction, serum cholesterol level, urinalysis proteinuria, urinalysis hematuria, and serum creatinine level. Compared with persons who had normal weight (BMI, 18.5 to 24.9 kg/m2), the adjusted relative risk for ESRD was 1.87 (95% CI, 1.64 to 2.14) for those who were overweight (BMI, 25.0 to 29.9 kg/m2), 3.57 (CI, 3.05 to 4.18) for those with class I obesity (BMI, 30.0 to 34.9 kg/m2), 6.12 (CI, 4.97 to 7.54) for those with class II obesity (BMI, 35.0 to 39.9 kg/m2), and 7.07 (CI, 5.37 to 9.31) for those with extreme obesity (BMI > or = 40 kg/m2). Higher baseline BMI remained an independent predictor for ESRD after additional adjustments for baseline blood pressure level and presence or absence of diabetes mellitus. LIMITATIONS: Primary analyses were based on single measurements of exposures. CONCLUSIONS: High BMI is a common, strong, and potentially modifiable risk factor for ESRD.