Eight athletes operated on for knee injuries were followed with muscle biopsies before and at various intervals after surgery and immobilization. A statistically significant change of the muscle fiber distribution was found. The percentage type I fibers dropped from an average of 54% to 43%. One competitive cross-country skier showed a dramatic drop from 81% type I fibers at surgery to 58% type I fibers six weeks later. After beginning training he returned to 85% type I fibers. One athlete who had been operated and immobilized for long periods several times showed a drastic difference in fiber type distribution between his two thighs with 20% type I fibers in the injured leg and 69% type I fibers in his uninjured leg. After three years of training his fiber type composition in the injured leg returned toward the fiber type distribution of the uninjured thigh. It is evident that muscle fiber type composition can change. The most probable reason for this is that the drastic change from hard sports training to nearly complete immobilization influences both the muscle itself and its innervation and causes this change of fiber types.