The authors examined body mass index at middle age, body mass index in old age, and weight change between age 50 years and old age in relation to mortality in old age. The study population from the Established Populations for Epidemiologic Studies of the Elderly consisted of 6,387 whites age 70 years or older who experienced 2,650 deaths during the period 1982-1987. Mortality risk was highest for persons in the heaviest quintile of body mass index at age 50 (men, relative risk (RR) = 1.33, 95% confidence interval (CI) 1.13-1.57; women, RR = 1.31, 95% CI 1.12-1.53) compared with persons in the middle quintile. This pattern was reversed for body mass index in old age, with persons in the lowest quintile having the highest mortality risk (men, RR = 1.40, 95% CI 1.19-1.65; women, RR = 1.38, 95% CI 1.17-1.63) relative to persons in the middle quintile. This reversal was explained, in part, by weight change. Compared with persons with stable weight, those who lost 10 percent or more of body weight between age 50 and old age had the highest risk of mortality (men, RR = 1.69, 95% CI 1.45-1.97; women, RR = 1.62, 95% CI 1.38-1.90). Exclusion of participants who lost 10 percent or more of their weight and adjustment for health status eliminated the higher risk of death associated with low weight. The inverse association of weight and mortality in old age appears to reflect illness-related weight loss from heavier weight in middle-age. Weight history may be critical to understanding weight and mortality relations in old age.