In this study we measured the effect on the muscle strength in the thigh in connection with diagnostic arthroscopy with central approach, arthroscopy with arthroscopic operation of bucket-handle ruptures, flap tears, total arthroscopic meniscectomy, and open meniscectomy. Thirty-seven patients were tested preoperatively and 1, 4 and 8 weeks postoperatively with a Cybex II isokinetic dynamometer. The extensor muscles of the knee joint were most affected. The flexor muscles were generally recovered in all groups within a month. Diagnostic arthroscopy and arthroscopic operations for bucket-handle ruptures showed no significant effect on the thigh muscle torque. With the other arthroscopic operations there was a 40% decrease of quadriceps function 1 week after operation. After 8 weeks normal function returned. Open meniscectomy had a much more negative effect on the quadriceps muscle with a decrease of about 70% after 1 week. After 8 weeks, the rehabilitation was still not complete.