In a retrospective review of 8428 hospital admissions, the relationship between age, sex, disease category, body mass index, and mortality during hospitalization was examined. Records were analyzed for adult admissions whose principal diagnosis fell into one of three categories: malignant disease, heart and cerebrovascular disease, and other diseases. In this study, age, disease category, and body mass index were predictors of survival; sex and race were not. Predicted mortality calculated by logistic regression was greatest at the extremes of body weight in all age groups and in each disease category describing a U-shaped relationship. Obesity was associated with higher mortality only when subjects were 100% or more overweight, whereas being at or below ideal weight was usually associated with increased mortality. Lowest mortality occurred at moderate overweight. The deleterious effects of extremes of body weight take on increasing importance the older the age of the patient. Underweight seems to be a more important predictor of mortality than overweight in older hospitalized subjects. The higher mortality in thin patients could not be explained by weight loss between hospitalizations.