Age-related changes in body composition, namely a decrease in fat-free mass (FFM) and bone mineral content (BMC) and a reciprocal increase in fat mass, are often observed in general populations. Dual-energy X-ray absorptiometry (DXA) makes it possible to analyze BMC, bone-free lean tissue mass (LTM), and fat mass more precisely and easily. To examine the changes in body composition with advancing age, percent body fat (%fat), BMC, LTM, and fat mass were measured in 79 adult men (48.8±19.9yr) and 134 adult women (50.8±19.3yr) using DXA (Lunar DPX-L). The subjects were divided into 7 subgroups according to chronological age. The highest %fat was 22.3±5.4% (70-79yr) in men, and 32.4±7.1% (60-69yr) in women. The mean peak BMC was observed at the age of 20-29 years in both men and women, being 3.3±0.5 kg and 2.6±0.3 kg respectively. Fat mass and %fat increased significantly (P<0.05) from age 20 (21.7±3.9%) to age 60(32.4±7.1%), and then decreased in women. Fat mass and %fat gradually increased (P<0.05) with advancing age, but not to a significant extent in men. BMC and LTM decreased significantly with advancing age in both sexes. The cumulative loss of mean BMC was greater in women than in men, but LTM showed on inverse trend. Characteristic changes in body composition associated with age in the region of the arms, legs and trunk varied considerably among body components. The mean fat mass of the arms and trunk was significantly larger at age 50-60 than at age 20 in women. The mean LTM of the legs decreased significantly with advancing age, but not in the arms and trunk. The fat mass and LTM of the trunk were higher than those of the arms and legs. However, BMC of the legs was higher than that of the trunk and arms in men, but similar in women. These results suggest that age-and gender-related differences in body components do exist in Japanese men and women.