The effect of aging on human gastrocnemius medialis (GM) muscle architecture was evaluated by comparing morphometric measurements on 14 young (aged 27–42 yr) and on 16 older (aged 70–81 yr) physically active men, matched for height, body mass, and physical activity. GM muscle anatomic cross-sectional area (ACSA) and volume (Vol) were measured by computerized tomography, and GM fascicle length ( Lf) and pennation angle (θ) were assessed by ultrasonography. GM physiological cross-sectional area (PCSA) was calculated as the ratio of Vol/ Lf. In the elderly, ACSA and Vol were, respectively, 19.1% ( P < 0.005) and 25.4% ( P < 0.001) smaller than in the young adults. Also, Lf and θ were found to be smaller in the elderly group by 10.2% ( P < 0.01) and 13.2% ( P < 0.01), respectively. When the data for the young and elderly adults were pooled together, θ significantly correlated with ACSA ( P < 0.05). Because of the reduced Vol and Lf in the elderly group, the resulting PCSA was found to be 15.2% ( P < 0.05) smaller. In conclusion, this study demonstrates that aging significantly affects human skeletal muscle architecture. These structural alterations are expected to have implications for muscle function in old age.