Objectives. We examined the relation between measures of body size and mortality in a predominantly White cohort of 8029 women aged 65 years and older who were participating in the Study of Osteoporotic Fractures. Methods. Body composition measures (fat and lean mass and percentage body fat) were calculated by bioelectrical impedance analysis. Anthropometric measures were body mass index (BMI; kg/m2) and waist circumference. Results. During 8 years of follow-up, there were 945 deaths. Mortality was lowest among women in the middle of the distribution of each body size measure. For BMI, the lowest mortality rates were in the range 24.6 to 29.8 kg/m2. The U-shaped relations were seen throughout the age ranges included in this study and were not attributable to smoking or measures of preexisting illness. Body composition measures were not better predictors of mortality than BMI or waist girth. Conclusions. Our results do not support applying the National Institutes of Health categorization of BMI from 25 to 29.9 kg/m2 as overweight in older women, because women with BMIs in this range had the lowest mortality.