The etiology of spontaneous hemarthrosis of the knee in elderly patients with osteoarthritis is still unknown. This report discusses six cases, one that was managed with conservative measures and five in which arthroscopic evaluation and treatment were performed. These patients were elderly individuals whose chief complaint was knee pain and swelling about the joint, without an obvious history of trauma. Each of the five operative patients underwent arthroscopic surgery after repeated hemorrhage into the joint that did not respond to serial aspirations of large amounts of blood and immobilization, using a knee immobilizer. The roentgenograms of each involved knee demonstrated lateral compartment osteoarthritis. Arthroscopic examination revealed a degenerative flap tear of the posterior horn of the lateral meniscus in the five operative cases. These patients underwent arthroscopic resection of the injured lateral meniscus and did not experience a recurrence of the hemarthrosis. The origin of the bleeding was most likely from the peripheral arteries of the posterior horn of the lateral meniscus.