Form March, 1983 to May, 1992 we have performed arthroscopy in 74 patients with traumatic hemarthrosis who had absent or negligible instability of the knee on physical examination, being concerned with establishing the role of early diagnostic knee arthroscopy in the evaluation of acute post-traumatic hemarthrosis. The results were as follows; 1. The age distribution was from 10 years to 63 years old, the mean age was 31.1 years old and the ratio of male to female was 3.6: l. 2. The mechanisms of injury were fall down in 28 cases (37.6%), traffic accident in 26 cases (35.1%) and sports injury in 20 cases (27.0%). 3. Arthroscopic findings were tear of the anterior cruciate ligament in 37, tear of joint capsule in 20, meniscus tear in 17, osteochondral fracture in 11, tear of the posterior cruciate ligament in 9 and plica rupture in 5 cases in order. 4. We experienced acute hemarthrosis of knee joint in 5 cases of ruptured mediopatellar plica which were impinged in the patellogemoral joint. 5. Early arthrscopy should be recommended to demonstrate the site and the severity of injuries for the accurate diagnosis and the treatment in the traumatic hemarthrosis of the knee joint, thus decrease the morbidity.