We tested the hypothesis that after the same amount of contraction-induced injury, skeletal muscles in old mice regenerate less well than muscles in young mice. Extensor digitorum longus (EDL) muscles in young and old mice were exposed to 15 min of lengthening contractions. The amount of injury was evaluated at 3, 7, 14, 28, and 60 days by measurements of maximum isometric tetanic force (Po) and number of fibers per cross section. When values 3 days after lengthening contractions were expressed as a percentage of control values, the Po (approximately 34%) and fiber number (approximately 80%) for muscles in old mice were not different from those in young mice, suggesting that muscles in old and young mice were injured to the same degree. By 28 days, injured muscles in young mice regained control values for Po and fiber number. In contrast, at 28 days, injured muscles in old mice recovered approximately 84 and approximately 87% of control values for Po and fiber number, respectively, and deficits in Po persisted at 60 days. We conclude that injured muscles regenerate less well in old mice than in young mice.