Knee extensor performance, in 17 subjects with chronic anterior cruciate ligament (ACL) tear, was investigated preoperatively and on four different occasions postoperatively, using isokinetic measurements and electromyography of single maximum and repetitive manoeuvres. Preoperatively maximum mechanical output was comparatively low (injured leg), deteriorating further by 50% at fourteen weeks postoperatively. Endurance also falls markedly. Thereafter knee-extensor performance improved successively, mostly during intensive training (14-20 weeks postoperatively) irrespective of the training programme used. After one year, maximum performance was still unequal but the injured leg had achieved the "normal" preoperative noninjured value. Fatiguability/endurance level improved over preoperative values. Muscular work/integrated EMG was stable while EMG/t increased. Twenty weeks postoperatively quadriceps area was decreased to 69%, cf. noninjured control. The early postoperative loss of performance was evidently caused by loss of muscle mass. Neuromuscular relearning appears to be a sizable factor in later recovery. Isokinetic training does not offer any specific advantage in the early muscular rehabilitation after ACL reconstruction.