BACKGROUND & AIMS Studies of obesity and diverticular complications are limited. We assessed the relationship between body mass index (BMI), waist circumference, and waist-to-hip ratio and diverticulitis and diverticular bleeding. METHODS A prospective cohort study of 47,228 male health professionals (40-75 years old) who were free of diverticular disease in 1986 (baseline) was performed. Men reporting newly diagnosed diverticular disease on biennial follow-up questionnaires were sent supplemental questionnaires. Weight was recorded every 2 years, and data on waist and hip circumferences were collected in 1987. RESULTS We documented 801 incident cases of diverticulitis and 383 incident cases of diverticular bleeding during 18 years of follow-up. After adjustment for other risk factors, men with a BMI >or= 30 kg/m(2) had a relative risk (RR) of 1.78 (95% confidence interval [CI], 1.08-2.94) for diverticulitis and 3.19 (95% CI, 1.45-7.00) for diverticular bleeding compared with men with a BMI of <21 kg/m(2). Men in the highest quintile of waist circumference, compared with those in the lowest, had a multivariable RR of 1.56 (95% CI, 1.18-2.07) for diverticulitis and 1.96 (95% CI, 1.30-2.97) for diverticular bleeding. Waist-to-hip ratio was also associated with the risk of diverticular complications when the highest and lowest quintiles were compared, with a multivariable RR of 1.62 (95% CI, 1.23-2.14) for diverticulitis and 1.91 (95% CI, 1.26-2.90) for diverticular bleeding. Adjustment for BMI did not change the associations seen for waist-to-hip ratio. CONCLUSIONS In this large prospective cohort, BMI, waist circumference, and waist-to-hip ratio significantly increased the risks of diverticulitis and diverticular bleeding.