We have investigated a series of 45 acute traumatic hemarthrosis of the knee between Jan. 1989 and Jan. 1993 at National Medical Center. All the patients who had normal findings on plain X-ray with hemarthrosis underwent arthroscopic examination under anesthesia. The arthroscopic results were as follows. 1. There were 14 cases(31%) of anterior cruciate ligament(ACL) tear, a case(2%) of avulsion fracture of tibial spine, 13 cases(29%) of lateral meniscus tear, 8 cases(18%) of medial meniscus tear, a case(2%) of osteochondral fracture of patella, and 8 cases(18%) showed no specific injury of internal structure. 2. The positive predictive value of Lachman and anterior drawer tests under anesthesia was 93% and 92%, respectively. 3. The positive predictive value of McMurray tests for medial and lateral menisci was 33% and 64%, respectively. 4. We concluded that a traumatic hemarthrosis indicated a significant knee injury and arthroscopy allowed a more accurate diagnosis of injury to joint structure.