For the past five years we have attempted to correct knee instability due to rupture of the posterior cruciate ligament with a procedure that employs a free graft of one-third of the patellar tendon with its tibial and patellar attachments. This procedure was done for chronic instability in thirty-three patients and was combined with primary repair of an acute mid-substance tear of the posterior cruciate ligament in fifteen patients. Moderate to severe articular injury of the medial femoral condyle was found at operation in 48 per cent of the patients with chronic injury. Seventy-one per cent of the patients for whom the interval between injury and ligament reconstruction was two to four years, and 90 per cent of those for whom the interval was more than four years, showed articular injury of the medial femoral condyle. Only 31 per cent of the patients, however, had preoperative radiographic findings that indicated femoral articular damage. Twenty-three of the twenty-five patients with a minimum follow-up of two years returned for evaluation. All of the ten patients who had had a repair and reconstruction of an acute ligament injury (whose average follow-up was forty-one months) had a static and functional result that was graded as good or excellent. Of the thirteen patients for whom surgery was done for chronic instability (whose average follow-up was thirty-one months), the over-all static and functional result was graded as good or excellent in eleven. These results indicate that the use of one-third of the patellar tendon for reconstruction in patients with acute mid-substance tears as well as in patients with symptomatic chronic instability of the posterior cruciate ligament is an effective procedure for achieving static and functional stability of the knee.