In nine patients with hemophilia and factor inhibitor (six with hemophilia A; three with hemophilia B), 19 joints were treated with radioactive synoviorthesis using Au-198. Ages ranged from 3 to 40 years. Synoviorthesis was performed when the antibody titer was low (< 10 Bethesda units), thus making hemostasis possible by factor administration for 2 to 4 days. On five occasions, radioactive synoviorthesis was performed simultaneously with tolerance induction according to the Malmö protocol. A bleeding free interval of more than 6 months was obtained in 11 joints, six of which remained bleeding free for more than a year. At long term followup (range, 18-182 months) five joints were rated good, one joint was fair, and 11 joints were poor. Although the results are inferior to those for patients with hemophilia without inhibitor, radioactive synoviorthesis should be considered because of its ease of performance and the definite decrease in joint bleeding frequency that it brings about. This is of particular interest in patients with hemophilia caused by factor inhibitor who otherwise are difficult to treat.