BACKGROUND AND PURPOSE: The association between obesity and stroke remains controversial, with earlier studies suggesting that differences might stem from heterogeneous stroke subtype compositions. The association between body mass index (BMI) and stroke subtypes was examined prospectively in a large cohort study. METHODS: A total of 234 863 Korean men aged 40 to 64 years without substantial weight loss over 4 years after baseline examination in 1986 were divided into 8 categories of BMI and were followed up between 1991 and 2000 for fatal and nonfatal stroke events. RESULTS: There was a positive association across the whole range of BMI and ischemic stroke, with a confounder-adjusted hazard of 11% (95% CI, 1.09 to 1.12) for 1 kg/m2 higher BMI. A J-shaped association was observed between BMI and hemorrhagic stroke; groups with a higher BMI than the reference category (22 to 23 kg/m2) had significantly increased risks. Full adjustment for confounders and variables potentially on the causal pathway (ie, blood pressure, blood glucose, and cholesterol) attenuated the association between BMI and stroke subtypes only for those with BMI greater than the reference category. Exclusion of deaths during the first 8 years or stratified analysis according to smoking habit did not change the relation between BMI and stroke subtypes. CONCLUSIONS: BMI is a risk factor for both ischemic and hemorrhagic stroke but shows different relationships with each. When the total burden of stroke is considered, there is an urgent need to find better ways of reducing the trend toward growing obesity in both Western and Asian countries.