SYNOVIAL hemangioma is an unusual cause of knee effusions, but must be considered in the individual who has recurrent nontraumatic knee effusions. The diagnosis is best made by eliciting a characteristic history and physical examination, finding grossly bloody fluid on joint aspiration, and pursuing with arthrography or arthroscopy and then arthrotomy. The case described here was of 20 years' duration. <h3>Report of a Case</h3> A 22-year-old man was seen with the latest in a long series of recurrent nontraumatic left knee effusions. His first effusion had occurred at age 2 years when a swollen, painful left knee had suddenly developed without inciting trauma. On examination, he had been found to have a temperature of 39 °C, a moderate effusion of his left knee, and marked tenderness, especially over the medial femoral condyle. Roentgenograms had been normal; WBC count, 11,400/cu mm, with a normal differential cell count; and erythrocyte sedimentation rate,