||CONTEXT: The association of body mass index (BMI) with cause-specific mortality has not been reported for the US population. OBJECTIVE: To estimate cause-specific excess deaths associated with underweight (BMI <18.5), overweight (BMI 25-<30), and obesity (BMI > or =30). DESIGN, SETTING, AND PARTICIPANTS: Cause-specific relative risks of mortality from the National Health and Nutrition Examination Survey (NHANES) I, 1971-1975; II, 1976-1980; and III, 1988-1994, with mortality follow-up through 2000 (571,042 person-years of follow-up) were combined with data on BMI and other covariates from NHANES 1999-2002 with underlying cause of death information for 2.3 million adults 25 years and older from 2004 vital statistics data for the United States. MAIN OUTCOME MEASURES: Cause-specific excess deaths in 2004 by BMI levels for categories of cardiovascular disease (CVD), cancer, and all other causes (noncancer, non-CVD causes). RESULTS: Based on total follow-up, underweight was associated with significantly increased mortality from noncancer, non-CVD causes (23,455 excess deaths; 95% confidence interval [CI], 11,848 to 35,061) but not associated with cancer or CVD mortality. Overweight was associated with significantly decreased mortality from noncancer, non-CVD causes (-69 299 excess deaths; 95% CI, -100 702 to -37 897) but not associated with cancer or CVD mortality. Obesity was associated with significantly increased CVD mortality (112,159 excess deaths; 95% CI, 87,842 to 136,476) but not associated with cancer mortality or with noncancer, non-CVD mortality. In further analyses, overweight and obesity combined were associated with increased mortality from diabetes and kidney disease (61 248 excess deaths; 95% CI, 49 685 to 72,811) and decreased mortality from other noncancer, non-CVD causes (-105,572 excess deaths; 95% CI, -161 816 to -49,328). Obesity was associated with increased mortality from cancers considered obesity-related (13,839 excess deaths; 95% CI, 1920 to 25,758) but not associated with mortality from other cancers. Comparisons across surveys suggested a decrease in the association of obesity with CVD mortality over time. CONCLUSIONS: The BMI-mortality association varies by cause of death. These results help to clarify the associations of BMI with all-cause mortality.