Radiosynovectomy (RSV) is a local intraarticular injection of radionuclides in colloidal form for radiotherapy. First used by Fellinger et al. in 1952, the technique has now been applied for more than 50 y for treatment of resistant synovitis of individual joints after failure of long-term systemic pharmacotherapy and intraarticular steroid injections. RSV relieves pain and inflammation from rheumatoid arthritis (RA), for which it initially was used, and is accepted as an alternative to surgical synovectomy in cases of RA or other inflammatory arthropathies such as osteoarthritis and hemophiliac arthropathy. A good understanding of the clinical pathophysiology of the disease processes is mandatory, and close interdisciplinary collaboration with other clinicians who diagnose and treat patients is strongly suggested. Reported success rates range from 40% to 90% for the different joints and underlying diseases. A few well-designed prospective double-blind trials have evaluated the efficacy of RSV and justified the procedure as a viable option for treating chronic synovitis in RA or secondary to inflammatory arthropathies. In comparison with surgical synovectomy, RSV produces equivalent results, costs less, allows the patient to remain ambulatory, and is repeatable. RSV has to be considered the initial procedure of choice for the treatment of patients with hemarthrosis in hemophilia. In addition, local instillation of radiopharmaceuticals can effectively reduce effusions after implantation of a prosthesis.