Background and Purpose-Previous studies have evaluated the association of body mass index (BMI) with the risk of all-cause and cardiovascular disease mortality among diabetic patients, and results were controversial. No studies have focused on the association between BMI and stroke risk among diabetic patients. We aimed to examine the association of BMI with stroke risk among diabetic patients. Methods-We performed a prospective cohort study with 29 554 patients with type 2 diabetes mellitus. Cox proportional hazards regression models were used to estimate the association of different levels of BMI with stroke risk. Results-During a mean follow-up period of 8.3 years, 2883 participants developed stroke (2821 ischemic and 109 hemorrhagic). The multivariable-adjusted (age, sex, race, smoking, income, and type of insurance) hazard ratios associated with different levels of BMI at baseline (18.5-24.9 [reference group], 25-29.9, 30-34.9, 35-39.9, and >= 40 kg/m(2)) were 1.00, 0.86, 0.83, 0.76, and 0.70 (P-trend <0.001) for total stroke, 1.00, 0.87, 0.85, 0.78, and 0.72 (P-trend <0.001) for ischemic stroke, and 1.00, 0.76, 0.72, 0.54, and 0.53 (P-trend=0.034) for hemorrhagic stroke, respectively. When we used an updated mean or the last visit value of BMI, the inverse association of BMI with stroke risk did not change. This inverse association was consistent among patients of different races, sex, ages, HbA1c levels, never and current smoking, and patients with and without using glucose-lowering, cholesterol-lowering, or antihypertensive agents. Conclusions-The present study demonstrates an inverse association between BMI and stroke risk among patients with type 2 diabetes mellitus.