We have compared the number of muscle fibers in the biceps brachii muscle (BB) of six old men (82.3 +/- 4.3 years) and six young men (21.2 +/- 1.9 years). Muscle fiber number was estimated by dividing the maximal area of the BB, determined with magnetic resonance imaging, by the mean fiber area of the BB determined in a muscle biopsy. The percentage of type II fibers in the BB ( approximately 60%) and the type I fiber area were not different between the groups. The BB area (-26%), type II fiber area (-24%), mean fiber area (-20%), and maximal voluntary contraction strength (MVC) of the elbow flexor muscles (-27%) were lower in the old than young group. However, the estimated number of muscle fibers was not significantly different between the young (253000) and old (234000) men. Consequently, the smaller BB area of the old men could be explained primarily by a smaller type II fiber size. These findings suggest that old age is not associated with a reduced number of muscle fibers in the BB. The relative contribution of a reduction in fiber number to age-related muscle atrophy may be muscle-dependent.