In their study of deaths associated with underweight, overweight, and obesity, Dr Flegal and colleagues1 conclude that excess mortality due to obesity and overweight is much lower than previously reported. We believe that their analysis is flawed and misleading. A major challenge in such studies is that low weight is often due to underlying chronic disease, which may exist for many years before death. Thus, lean persons are a mix of smokers, healthy active persons, and those with chronic illness (due to the direct effects of disease on weight and sometimes purposeful weight loss motivated by diagnosis of a serious illness). Their analysis does not successfully disentangle this diverse group.