Despite the progress in the treatment of rheumatoid arthritis (RA), many patients continue to suffer from persistent and painful synovitis. We assessed the clinical results of 64 intraarticular injections of Yttrium 90 in the knee and 56 injections of Rhenium 186 in the wrist in 71 patients with RA, older than 40 years, without relief of synovitis after six months of systemic or local treatment and with a radiological stage I or II of their joints. We obtained good results in 75% of knees and 100 of wrists during a follow up period of 8 to 60 months. A repeated histological examination of the synovial membrane of 14 joints in which good results were obtained disclosed a reduction in inflammation and transitory synovial cell hyperplasia that ended in a dense fibrosis. It is concluded that radiation synovectomy continues to be an effective therapy for selected patients with RA and synovitis. The observed histopathological changes may aid the interpretation of magnetic resonance imaging of the treated joints.