The purpose of the study was to compare the hamstring and quadriceps isokinetic results 6 months postoperatively in patients having patellar tendon or hamstring anterior cruciate ligament (ACL) reconstruction. The study group was comprised of 106 randomly selected patients who had ACL reconstruction with either autogenous patellar tendon (PT), semitendinosus (ST), or semitendinosus and gracilis (ST/G). Hamstring and quadriceps isokinetic strength were assessed at 180 degrees/sec and 300 degrees/sec with the results of the operatively treated leg expressed as a percent compared with the nonoperative leg. The mean results for knee extension at 180 degrees/sec were 68.3%, 74.3%, and 78.1%; and at 300 degrees/sec were 70.7%, 76.7%, and 81.7% for PT, ST, and ST/G, respectively. The mean results for knee flexion at 180 degrees/sec were 86.1%, 80.6%, and 81.7%; and at 300 degrees/sec were 77.6%, 79.1%, and 75.6% for PT, ST, and ST/G, respectively. No statistically significant differences were found in regard to knee extension or flexion strength when evaluating the different tissue sources. The results show that selection of autogenous hamstring or PT used for ACL reconstruction should not be based solely on the assumption of the tissue source altering the recovery of quadriceps and/or hamstring strength. In addition, a majority of the patients had not achieved adequate strength to safely partake in unlimited activities at 6 months postoperatively.