Twelve male subjects with recreational resistance training backgrounds completed 12 wk of intensified resistance training (3 sessions/wk; 8 exercises/session; 3 sets/exercise; 10 repetitions maximum/set). All major muscle groups were trained, with four exercises emphasizing the forearm flexors. After training, strength (1-repetition maximum preacher curl) increased by 25% (P < 0.05). Magnetic resonance imaging scans revealed an increase in the biceps brachii muscle cross-sectional area (CSA) (from 11.8 +/- 2.7 to 13.3 +/- 2.6 cm2; n = 8; P < 0.05). Muscle biopsies of the biceps brachii revealed increases (P < 0.05) in fiber areas for type I (from 4,196 +/- 859 to 4,617 +/- 1,116 microns2; n = 11) and II fibers (from 6,378 +/- 1,552 to 7,474 +/- 2,017 microns2; n = 11). Fiber number estimated from the above measurements did not change after training (293.2 +/- 61.5 x 10(3) pretraining; 297.5 +/- 69.5 x 10(3) posttraining; n = 8). However, the magnitude of muscle fiber hypertrophy may influence this response because those subjects with less relative muscle fiber hypertrophy, but similar increases in muscle CSA, showed evidence of an increase in fiber number. Capillaries per fiber increased significantly (P < 0.05) for both type I (from 4.9 +/- 0.6 to 5.5 +/- 0.7; n = 10) and II fibers (from 5.1 +/- 0.8 to 6.2 +/- 0.7; n = 10). No changes occurred in capillaries per fiber area or muscle area. In conclusion, resistance training resulted in hypertrophy of the total muscle CSA and fiber areas with no change in estimated fiber number, whereas capillary changes were proportional to muscle fiber growth.