ObjectiveTo determine if body mass Index (BMI = weight [kg]/height [m]2), predictive of mortality in longitudinal epidemiologic studies, was also predictive of mortality in a sample of seriously ill hospitalized subjects.DesignProspective, multicenter study.SettingFive tertiary care medical centers in the United States.Patientsor=to18 yrs of age who had one of nine illnesses of sufficient severity to anticipate a 6-month mortality rate of 50% were enrolled at five participating sites in the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatments (SUPPORT).InterventionsNone.Measurements and Main Resultsor=to85th percentile) was not significantly related to risk of mortality.ConclusionsBody mass index, a simple anthropometric measure of nutrition employed in community epidemiologic studies, has now been demonstrated to be a predictor of mortality in an acutely ill population of adults at five different tertiary centers. Even when controlling for multiple disease states and physiologic variables and removing from the analysis all patients with significant prior weight loss, a body mass index below the 15th percentile remained a significant and independent predictor of mortality. Examination of patient vs. proxy data did not change the results. Future studies examining variables predictive of mortality should include body mass index, even in acutely ill populations with a poor probability of survival. (Crit Care Med 1997; 25:1962-1968)