BACKGROUND The prevalence of adult obesity has increased in recent decades. It is important to predict the long-term effect of body weight, and changes in body weight, in middle age on longevity and Medicare costs in older ages. METHODS The relationships between individuals' characteristics in middle age and subsequent Medicare costs and mortality were estimated from the linkage of the National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study to Medicare administrative records (1991-2000) and mortality information (1971-2000). We predicted longevity and lifetime Medicare costs via simulation for 45-year-old persons by body weight in 1973 and changes in body weight between 1973 and 1983. RESULTS Obese 45-year-olds had a smaller chance of surviving to age 65 and, if they did, incurred significantly higher average lifetime Medicare costs than normal-weight 45-year-olds ($163,000 compared with $117,000). Those who remained obese between ages 45 and 55 in 1973 to 1983 incurred significantly higher lifetime Medicare costs than those who maintained normal weight. Other weight change categories did not differ significantly from those who maintained normal weight in terms of life expectancy at age 65, but overweight and obese people who lost weight had less chance of surviving to age 65 and the lowest estimated life expectancies thereafter. CONCLUSIONS Chronic obesity in middle age increases lifetime Medicare costs relative to those who remained normal weight. As the survival of obese persons improves, it is possible that Medicare costs may rise substantially in the future to meet the health care needs of today's obese middle-aged population. Thus, active engagement by both the private and public sectors to prevent and to reduce obesity are critically needed.