Low body mass index (BMI) is a risk factor for fracture, but little is known about the association between high BMI and fracture risk. We evaluated the association between BMI and fracture in the Osteoporotic Fractures in Men Study (MrOS), a cohort of 5995 US men 65 years of age and older. Standardized measures included weight, height, and hip bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA); medical history; lifestyle; and physical performance. Only 6 men (0.1%) were underweight (<18.5kg/m2); therefore, men in this category were excluded. Also, 27% of men had normal BMI (18.5 to 24.9kg/m2), 52% were overweight (25 to 29.9kg/m 2), 18% were obese I (30 to 34.9kg/m2), and 3% were obese II (35 to 39.9kg/m2). Overall, nonspine fracture incidence was 16.1 per 1000 person-years, and hip fracture incidence was 3.1 per 1000 person-years. In age-, race-, and BMD-adjusted models, compared with normal weight, the hazard ratio (HR) for nonspine fracture was 1.04 [95% confidence interval (CI) 0.87-1.25] for overweight, 1.29 (95% CI 1.00-1.67) for obese I, and 1.94 (95% CI 1.25-3.02) for obese II. Associations were weaker and not statistically significant after adjustment for mobility limitations and walking pace (HR=1.02, 95% CI 0.84-1.23, for overweight; HR=1.12, 95% CI 0.86-1.46, for obese I, and HR=1.44, 95% CI 0.90-2.28, for obese II). Obesity is common among older men, and when BMD is held constant, it is associated with an increased risk of fracture. This association is at least partially explained by worse physical function in obese men. © 2011 American Society for Bone and Mineral Research.