The aim of this study was to compare the treatment results between radiosynoviorthesis and reradiosynoviorthesis of knees. Before the radiosynoviorthesis, an ultrasonography, X-ray, and three-phase bone scintigraphy were done. The treatment effect can be expected if a synovitis is proved by these examinations. To knees, 200 MBq of the yttrium citrate was injected for the first radiosynoviorthesis or for reradiosynoviorthesis. After an application, it is possible to do the scintigraphic examination, when information about a tracer distribution in joints is obtained. The treatment effect was evaluated by the clinical examination, the ultrasonography, and the three-phase bone scintigraphy with some lapse of time. If the effect of the radiosynoviorthesis was not satisfying, it could be repeated no sooner than 6 months later. Among our patients we had a high percentage of the repeated radiosynoviorthesis. The authors applicated the yttrium citrate to 1243 knees. A rate between single radiosynoviorthesis and reradiosynoviorthesis was 11:8. Repeated radiosynoviorthesis were as effective as the initial ones, and their repeated use does not decrease the expected therapeutic effect.