This prospective study evaluated the athletes with chronic instability of the knees using the expanded polytetrafluoroethylene (PTFE) prosthetic ligament as a substitute for the ACL and PCL preoperatively and at regular intervals during the post-operative period. Thirty Gore-Tex ACL and four Gore-Tex PCL reconstruction were performed. All patients had chronic instability of the knee joint for a duration of one month to five years (average 15.7 months). There were thirty-one males and three females, most of them were football players (63%). The average follow-up was 23 months postoperation. Ninety-seven per cent of the patients had satisfactory results. Objectively the instability pattern was improved in 97 per cent of the knees and subjectively, no pain persisted post operatively. Rejection occurred in one patient. The results indicated excellent stability of the affected knee joint. However, longevity of the prosthesis under the wide variety of loading conditions in athletes remains to be determined by continuing clinical study.