Arthroscopy of the knee joint is an essential diagnostic procedure because of high diagnostic accuracy and valuable method for a rational treatment. We were concemed with establishing the role of early diagnostic knee arthroscopy in the evaluation of acute post-traumatic hemarthrosis. From May, 1986 to May, 1990 arthroscopy was performed in 50 patients with acute hemarthrosis who could not show gross instability of the knee during physical examination. The results were as follows; 1. Males were predominant (40 males and 10 females) and the mean age was 29.7 years old. 2. The mechanisms of injury were sports injury in 19 cases (38%), fall down in 16 cases (22%), and traffic accident in 13 cases (26%). 3. Out of the 50 cases there were 32 cases (64%) of peripheral meniscus tear, 30 cases (60%) of partial or complete tear of the anterior cruciate ligament, 5 cases (10%) of osteochondral fracture, and 4 cases (8%) of other injuries. And concomitant injuries associated with other intraarticular structures were 19 of 30 cases (63%) with anterior cruciate ligament rupture, 17 of 32 cases (53%) with peripheral meniscus tear, and 4 of 5 cases (90%) with osteochodral fracture. 4. Treatment after arthroscopy in 50 cases was performed operative procedure under arthroscopy in 12 cases, open arthrotomy after arthroscopy in 8 cases and conservertive treatment in 30 cases. 5. Among the torn A.C.L. that was verified by arthroscope, physical examination before and after anesthesia was performed in 10 cases. The positive anterior drawer and Lachman test increase from 2 cases (20%) to 6 cases (60%) and 4 cases (40%) to 9 cases (90%) in the 10 cases. 6. Accuracy of clinical diagnosis by arthroscopy was 56% in the isolated lesion, 25% in the combined lesion. 7. In the evaluation of the post traumatic hemarthrosis of the knee, it is difficult to demonstrate the site and severity of the injuries with clinical exmination alone. Therefore for making a correct clinical diagnosis, arthroscopy is recommended in case of traumatic hemarthreosis of the knee.