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.