Knowledge of the association between body mass index (weight (kg)/height (m)(2)) and premature death in young adulthood is very limited, especially for specific causes of death. Using the US National Health Interview Survey linked mortality files, the authors examined the relation between body mass index and premature death from all causes, cardiovascular disease (CVD), and cancer among 112,328 persons aged 18-39 years who participated in the National Health Interview Survey in the years 1987, 1988, and 1990-1995. During an average of 16 years of follow-up (ending on December 31, 2006), there were 3,178 deaths: 573 from CVD and 733 from cancer. Hazard ratios and 95% confidence intervals were estimated using multivariate proportional hazards models adjusting for age, gender, race/ethnicity, education, and smoking status. In analyses restricted to participants who had never smoked, the hazard ratios for death from all causes were 1.07 (95% confidence interval (CI): 0.91, 1.26) for overweight participants, 1.41 (95% CI: 1.16, 1.73) for obese participants, and 2.46 (95% CI: 1.91, 3.16) for extremely obese participants, compared with those of normal weight. Monotonically increasing risks for excess body weight were also observed for deaths from cancer and CVD. The associations found in this young cohort were much stronger than those in middle-aged or older populations.