Patients suffering therapy-resistant postoperative synovialitis after receiving knee endoprosthesis implantation (TEP) have been treated with radiosynoviorthesis (RSO) combined with Yttrium90-citrate. We compared the data with the results of RSO applied to patients with rheumatoid arthritis or activated gonarthrosis being treated with Yttrium-citrate. 35 patients with 38 treated knee joints were examined. Altogether, 47 radiosynoviortheses were applied, followed by an average checkup period of 7.7 months. For an overall evaluation of therapy success rate we have developed an evaluation score system (RSO index) following the points system by Kerschbaumer, which, besides objective clinical parameters such as bruises and overheating etc., takes the subjective factor pain into account. Additionally, we registered parameters such as swellings, agility and pain frequency. The results have been evaluated with regard to influence of diagnosis, patient age, applied activity level, x-ray progression and therapy method (combined therapy of open or arthroscopic synovectomy and RSO versus exclusive therapy with Yttrium-citrate). According to the RSO index, therapy success rate of TEP patients was slightly worse than success rate of rheumatoid arthritis and activated gonarthrosis patients. 64.7 % of TEP knee joints showed results ranging from satisfactory to very good, patients with rheumatoid arthritis and activated gonarthrosis reached 81.8 % respectively 75 % of all knee joints, however there was no significant difference between the three diagnosis groups. Considering single parameters such as knee-joint bruises, swellings and pain intensity, patients with rheumatoid arthritis and activated gonarthrosis showed significantly better results than patients with knee endoprosthesis implantation. The success rate of radiosynoviorthesis depended on the x-ray progression. Patients without or only a few radiological changes achieved considerably better results. Diagnosis, patient age, applied activity level and therapy scheme did not have a significant influence on therapy results. On the basis of our results, we conclude that radiosynoviorthesis combined with Yttrium-citrate is a useful and efficient method for the treatment of therapy-resistant synovialitis after knee prosthesis implantation.