Although satisfactory results have been obtained with conventional synovectomy of the knee, there are frequent complications, and a long period of rehabilitation is necessary after this surgery. We performed arthroscopic synovectomy of the knees of 30 patients (33 knees), 22 of whom had rheumatoid arthritis. These patients underwent the procedure after failure to respond to an intraarticular injection of either osmic acid or yttrium 90. The operated knee was moved 3-5 hours after the arthroscopy, and the patients walked the next day. No rehabilitation was needed. The followup period ranged from 6 months to 36 months, with an average of 17.7 months. Except for 1 patient with severe arthritis, all patients experienced improvement. Results were rated as "good" or "very good" in 27 knees. Six months after synovectomy, the patients were asked to rate their improvement; the mean +/- SEM degree of improvement was 79.1 +/- 22.9%. All patients had significant improvement in the range of motion of the knee. One patient required manipulation of the knee (while under anesthesia) soon after the arthroscopy. There were no other complications. There was no detectable radiographic evidence of disease progression in 24 patients who were seen 1 year after the procedure or in 9 patients who were seen 2 years after the arthroscopy. Thus, arthroscopic synovectomy appears to be an effective and simple treatment for chronic knee synovitis, and has a low morbidity rate.