A 24-year-old woman was diagnosed with monoarticular spontaneous nontraumatic knee hemarthrosis at age 9 years. Following multiple scans and arthroscopies, a diagnosis of cavernous synovial hemangioma was made radiographically and histologically 15 years later. This was treated surgically with excellent results. Preoperative magnetic resonance imaging (MRI) and Doppler ultrasound are recommended for diagnosis and surgical planning, with the use of a dual coagulation shaver for definitive excision of the lesion. Follow-up should be conducted 6 and 12 months postoperatively with MRI and Doppler ultrasound to determine recurrence.