The association between body mass index (weight (kg)/height (m)(2)) and mortality may be confounded by preexisting illness. A method commonly used to control for this confounding is the exclusion of participants who have certain diseases at baseline and/or those who die during the early years of follow-up. The authors used data from the Atherosclerosis Risk in Communities (ARIC) Study (n = 14,088) to determine whether participants identified by these criteria had different changes in body mass index than other participants. Weight change was measured over a 3-year interval between study entry (1987-1989) and reexamination (1990-1993), and information on vital status was collected over the subsequent 5 years. Mean change in body mass index was -0.54 (95% confidence interval (CI): -0.90, -0.12) among participants who died in the first year of follow-up, -0.03 (95% CI: -0.18, 0.12) among those who died in the first 4 years of follow-up, and 0.36 (95% CI: 0.33, 0.39) among those who survived for at least 5 years. Participants who died during the first 4 years were over twice as likely as survivors to have changed from the obese category (body mass index >or=30) to the nonobese category (odds ratio = 2.14; 95% CI: 1.44, 3.17). Mean change in body mass index prior to baseline was not different among ill participants compared with those who were healthy, but the odds of converting from obese to nonobese were higher in ill participants than in healthy ones (odds ratio = 1.29; 95% CI: 1.01, 1.67).