The structural and functional characteristics of the elbow flexors in five elderly males were studied before and after 12 wk of heavy-resistance training. Muscle volume and cross-sectional area of two of the elbow flexor (biceps brachii and brachialis) muscles were determined by magnetic resonance imaging. Mean muscle fiber area, percent fiber distribution, and collagen and noncontractile tissue densities were determined on histological sections from needle biopsies. Isokinetic strength of the elbow flexors was measured at velocities between 60 and 300 degrees/s. Muscle volume and cross-sectional area of the biceps brachii and brachialis significantly increased by 13.9 and 22.6%, respectively, after the training program. A preferential hypertrophy of type II fibers (37.2%) was observed. Significant increases in peak torque were observed at all the tested velocities. The amount of work a subject could perform during a 25-repetition test at 240 degrees/s increased by 41% after training. These results demonstrate that the skeletal muscles of elderly individuals can adapt to heavy-resistance exercise and do so by increases in both muscle size and strength.