The authors have examined the relation between the Quetelet body mass index (BMI) and 26-year risk of all-cause mortality in a population of 12,576 non-Hispanic while, Seventh-day Adventist women (ages 30-74 years) who never smoked. Mortality risk for each BMI quintile (I, < 21.3 kg/m2; II, 21.3-22.9 kg/m2; III, 23.0-24.8 kg/m2; IV, 24.9-27.4 kg/m2; and V, > 27.4 kg/m2) was determined from a proportional hazard regression with adjustment for age and other covariables. In this population, the overall BMI-mortality relation showed dependence on age, duration of follow-up, and baseline indicators of preexisting illness (weight fluctuation, history of major chronic disease, and severe physical complaints). Therefore, the analysis focused on women with no indicators of preexisting illness, and risk estimates were stratified by age at baseline and duration of follow-up. Among middle-aged women (ages 30-54 years), the authors found a weak linear relation during years 1-8 (median attained age, 51 years), a significant linear relation during years 9-14 (median attained age, 57 years), and a significant nonlinear (U-shaped) relation during years 15-26 (median attained age, 68 years). Among older women (ages 55-74 years), they found a significant nonlinear (U-shaped) relation during years 1-8 (median attained age, 71 years) and significant linear relations during years 9-14 (median attained age, 77 years) and years 15-26 (median attained age, 87 years). These findings implicate overweight as a risk factor for fatal disease among women throughout adulthood and raise the possibility that lean, apparently healthy, middle-aged women may experience a higher risk of death during old age due to their lower body weight.