165Dy Hydroxide Macroaggregates(165Dy HMA) has a short half life(2.3 hours) and a size range of 3-5µm that give the advantage of reduced leakage and a shorter hospital stay. This report will show the results of a prospective open study on the efficacy and safety of 165Dy HMA in 178 knees of 141 patients with chronic synovitis refractory to conventional antirheumatic therapy. The final global assessment was classified as good, fair or poor. Extra-articular leakage of 165Dy HMA was determined by the scintigraphic evaluation of liver, groin and knee joints. The optimum radiation dose was 250 mCi. The mean follow up periods were 32.4(14-112) weeks. Thirty seven percent of the knees showed good results, 48% fair results and 15% poor results. In the knees with stage I radiographic changes, 82% showed improvement including 32% of the patients with good results. In the knees with stage II radiographic changes, 90% showed improvement including 42% of the patients with good results. The mean period of improvement for the 158 knees that responded to treatment was 41.4(24-106) weeks. Leakage of radioactivity from the injected joint was minimal. Adverse reactions were rare(radiation burn : 4 cases, transient postinjection swelling : 14 cases). In conclusion, 165Dy HMA radiation synovectomy is a safe and useful therapy for chromic synovitis of the rheumatoid knees.