The overall aim of this study was to assess the effects of aging on the satellite cell population. Muscle biopsies were taken from the tibialis anterior muscle of healthy, moderately active young (age range, 20-32 years; n = 31) and elderly (age range, 70-83 years; n = 27) women and men with comparable physical activity pattern. Satellite cells and myonuclei were visualized using a monoclonal antibody against neural cell adhesion molecule and counterstained with Mayer's hematoxylin. An average of 211 (range, 192-241) muscle fibers were examined for each individual. Compared with the young women and men, the elderly subjects had a significantly lower (P < 0.011) number of satellite cells per muscle fiber but a significantly higher (P < 0.004) number of myonuclei per muscle fiber. The number of satellite cells relative to the total number of nuclei [satellite cells/(myonuclei + satellite cells)] was significantly lower in the elderly than in the young women and men. These results imply that a reduction in the satellite cell population occurs as a result of increasing age in healthy men and women.