Background: With aging, muscle mass decreases but fat mass increases, The purpose of this study was to investigate the relationship of changes in body fat and muscle mass with endocrine function. Methods: Fasting serum levels of hormones were determined in 52 normal weight and 100 overweight-obese Korean men with a wide age range(21-60yr). Adipose tissue and muscle areas were calculated from computed tomography scans made at two body levels, umbilicus and mid portion of thigh. Results: Total abdominal fat areas did not vary with aging in normal weight and overweight-obese men, while visceral fat areas showed an increase at age 41-60, compared with age 21-30. Thigh muscle mass and serum testosterone levels remained relatively stable before age 40 and declined rapidly thereafter. At age 41-50, the mean values of serum testosterone and free androgen index were only 72% and 49% of those at age 21-30, respectively. The serum levels of insulin and IGF-1 declined by 20% and 33% between age 21-30 and age 51-60, respectively, Increased visceral fat accumulation was associated with reduced serum levels of testosterone and IGF-1, and elevated levels of insulin. Thigh muscle mass was positively correlated with serum levels of testosterone, free androgen index and insulin. Fasting serum levels of testosterone were correlated negatively with serum insulin. Conclusion: Korean men with aging showed an increase in visceral fat and a rapid decrease in thigh muscle mass although body weight and total abdominal fat mass were maintained. These changes in regional body fat and muscle mass might result from reduced serum levels of testosterone and IGF-1.