A case of spontaneous recurrent hemarthrosis--due to developed hypertrophied synovium--after total knee arthroplasty is reported. The patient was successfully treated with radiosynovectomy. The first hemorrhage occurred 18 months after the total knee arthroplasty. Several similar episodes followed over a period of 4 years. Because conservative treatment failed to control the bleeding, an arthroscopic lavage was performed, which revealed the existence of proliferative synovium. A significant part of the hypertrophic tissue was excised with the use of a thermocoagulator. However, 1 month later, another episode of hemarthrosis occurred. As a final step before reoperation, the patient was treated with intra-articular injection of ytrium 90. Eighteen months later, she remains symptom-free, is very satisfied with the result, and reports no new episode of hemarthrosis.