In this long term retrospective study of radiation synovectomy with Yttrium-90 (Y90), we evaluated the results of 164 applications in 82 patients with RA, OA with synovitis, ankylosing spondylitis and psoriatic arthritis. Radiation synovectomy with Y90 has an overall success rate of approximately 50% and is therefore an effective alternative to surgical synovectomy in chronic synovitis which fails to respond to conservative treatment. Elbow and knee responded significantly better than shoulder and ankle joints. Patients with radiological stages from 0 to 2 showed a significantly better success rate than those with stage 3 changes. In responders, repeat therapy for recurrence of symptoms or treatment of a symptomatic corresponding symmetrical joint is advisable. Repeat therapy in a previous non-responder is associated with an unacceptably high failure rate. Therefore, when a joint fails to respond after 6 months, arthroscopy should be performed to evaluate further treatment procedures. A successful result was found in only 11 of 25 joints treated with arthroscopic synovectomy followed by radiation synovectomy within 2 weeks, indicating no benefit of this combination.