Pigmented villonodular synovitis (PVS) is a rare disease involving the synovial membrane and causing symptoms similar to arthritis. In diffuse PVS, recurrence is not uncommon after surgery. We report a male patient with PVS of the knee that recurred 1 year and 10 months after initial synovectomy. He was again operated on and then given adjuvant radiation therapy (RT) with a total dose of 34.5 Gy in 15 fractions beginning 39 days postoperatively. RT relieved his symptoms even though an MRI revealed persistent disease. RT may be beneficial for selected patients with diffuse PVS.