The authors studied distributions of body weight for height, change in body weight with age, and the relationship between body mass index and mortality among participants in the Epidemiologic Follow-up Study of the first National Health and Nutrition Examination Survey (NHEFS) (n = 14,407), a cohort study based on an representative sample of the U.S. population. Percentiles of body weight for height according to age and sex are presented. Cross-sectional analyses of body weight suggest that mean body weight increases with age until late middle age, then plateaus and decreases for older aged persons. However, longitudinal analysis of change in weight with age shows that younger persons in the lower quintile at baseline tend to gain more than those in the higher quintile. Older persons in the higher quintile at baseline have the greatest average loss in weight. The relationship of body mass index to mortality is a U-shaped curve, with increased risks in the lowest and highest 15% of the distribution. Increased risk of mortality associated with the highest 15th percentile of the body mass index distribution, as well as the highest 15% of the joint distribution of body mass index and skinfold thickness, is statistically significant for white women. However, the risk diminishes when adjusted for the presence of disease and factors related to disease. More noteworthy is the fact that there is a statistically significant excess risk of mortality for both race and sex groups in the lowest 15% of the body mass index distribution after adjusting for smoking history, and presence of disease. Those in the lowest 15% of the joint body mass index and skinfold thickness distribution, were also at increased risk. Risk of mortality for both men and women who have lost 10% or more of their maximum lifetime weight within the last 10 years is statistically significant, even when controlling for current weight. This study has replicated previously reported relationships, while correcting for several methodological issues.