Introduction People with haemophilia (PwH) with inhibitors have an increased risk of bleeding and early development of progressive arthropathy. Radiosynovectomy (RS) has been effective in dramatically reducing the frequency of haemarthroses. In the present study, the mid- and long-term results of the efficacy of RS in PwHs with inhibitors and prognostic factors that influence success and failure of RS were presented. Material and Method Radiosynovectomy was performed in 51 joints of 22 PwHs with inhibitors diagnosed with chronic haemophilic synovitis between January 2000 and December 2018. Two patients were lost to follow-up and four joints were excluded. Number of bleeding episodes within the pre- and post-treatment 6 months were documented. Treatment failure was defined as need for repeat RS injection. Results Results of 47 RS were analysed. The mean bleeding frequency of the joints was 11.2 +/- 6.2 (median 9) within the last 6 months in the pre-treatment evaluation. After the treatment, the mean bleeding frequency of the joints decreased to 1.2 +/- 2.8 (median 0) for first 6 months (P < .0001). The cumulative survival rate at 12 months was 87% and 78% at 36 months. The receiver operating characteristic (ROC) curve analysis revealed that cut-off points of 12 bleeding episodes within the last 6 months (sensitivity, 71.4; specificity, 81.8P = .0022) and an inhibitor titre of 63.4 BU (sensitivity, 57.1; specificity, 75.8;P = .31) were threshold levels for a predisposition for failure. Conclusion Radiosynovectomy is an effective and safe intervention in PwHs with inhibitors. Bleeding frequency is a prognostic marker for the success of RS treatment. Patients who have more than 12 bleeding episodes within the last 6 months before the RS treatment have a higher rate of failure.