Increasing prevalence of overweight/obesity and rapid aging of the US population have raised concerns of increasing health care costs, with important implications for Medicare. However, little is known about the impact of body mass index (BMI) earlier in life on Medicare expenditures (cardiovascular disease [CVD]-related, diabetes-related, and total) in older age.To examine relationships of BMI in young adulthood and middle age to subsequent health care expenditures at ages 65 years and older.Medicare data (1984-2002) were linked with baseline data from the Chicago Heart Association Detection Project in Industry (CHA) (1967-1973) for 9978 men (mean age, 46.0 years) and 7623 women (mean age, 48.4 years) (baseline overall age range, 33 to 64 years) who were free of coronary heart disease, diabetes, and major electrocardiographic abnormalities, were not underweight (BMI <18.5), and were Medicare-eligible (> or =65 years) for at least 2 years during 1984-2002. Participants were classified by their baseline BMI as nonoverweight (BMI, 18.5-24.9), overweight (25.0-29.9), obese (30.0-34.9), and severely obese (> or =35.0).Cardiovascular disease-related, diabetes-related, and total average annual Medicare charges, and cumulative Medicare charges from age 65 years to death or to age 83 years.In multivariate analyses, average annual and cumulative Medicare charges (CVD-related, diabetes-related, and total) were significantly higher by higher baseline BMI for both men and women. Thus, with adjustment for baseline age, race, education, and smoking, total average annual charges for nonoverweight, overweight, obese, and severely obese women were, respectively, 6224 dollars, 7653 dollars, 9612 dollars, and 12,342 dollars (P<.001 for trend); corresponding total cumulative charges were 76, 866 dollars, 100,959 dollars, 125,470 dollars, and 174,752 dollars (P<.001 for trend). For nonoverweight, overweight, obese, and severely obese men, total average annual charges were, respectively, 7205 dollars, 8390 dollars, 10,128 dollars, and 13,674 dollars (P<.001 for trend). Corresponding total cumulative charges were 100,431 dollars, 109,098 dollars, 119,318 dollars, and 176,947 dollars (P<.001 for trend).Overweight/obesity in young adulthood and middle age has long-term adverse consequences for health care costs in older age.