The studied muscle loss with aging, focusing on differences due to muscle group location. Muscle thickness at nine sites-forearm, upper arm anterior and posterior, abdomen, subscapular, thigh anterior and posterior, and lower leg anterior and posterior-were determined using a brightness-mode ultrasonography in 348 Japanese men aged 20 to 79 years. Only the upper arm anterior did not show a significant effect of age. For other sites, the starting age group that significantly decreased from 20-29 yr and 30-39 yr was 40-49 yr for the forearm and abdomen, 50-59 yr for the thigh anterior, 60-69 yr for the upper arm posterior, lower leg anterior and posterior, and subscapular, and 70-79 yr for the thigh posterior. The relative reduction between the 20-29 yr and 70-79 yr groups in the muscle thickness at the abdomen and thigh anterior were greater than those in the other sites. In addition, the upper arm and thigh showed a preferential loss with aging at the posterior and anterior sites. Thus, the present results indicated that the loss of muscle thickness with aging differed between body segments and between sites within the same segment. The reasons for the site-related differences in muscle loss with aging may be attributed to age-related changes in the patterns of loading to individual muscles and/or their activations, which are encountered during daily life.