Objectives: This study was performed to answer the following questions: First, does the knee examination under general anesthesia (GA) and arthroscopy allow a more accurate diagnosis? Second, what is the incidence and the differential diagnosis of injured knees with hemarthrosis. Methods: This retrospective study was performed on 350 cases from a single center with complaints of knee pain and swelling. Results: Seventy out of the 350 cases were selected based on inclusion and exclusion criteria. The average age was 30.6 years. The right knee was injured in 44.3% left knee in 55.7%. A professional sport level was in 17.1% a recreational was in 82.9%. Indirect injury caused 71% and 29% were caused by direct injury. Lachman test at emergency was positive in 38.6% negative in 38.6% difficult to perform in 17% not examined in 5.7%. Under GA, Lachman test was positive in 81.4% negative in 10% not examined in 8.6%. Under arthroscopy: 94.3% had anterior cruciate ligament (ACL) tears the ACL was intact in 5.7% completely ruptured in 78.6% and partially ruptured in 15.7%. Isolated total ACL rupture was in 31.4%. Isolated partial ACL rupture was in 15.7%. Posterior cruciate ligament (PCL) partial rupture was in 2.9%. Meniscal tear was in 71.4% lateral meniscal tear (LM) in 40% and medial meniscal tear (MM) in 31.4%. ACL + LM in 22.9% ACL + MM in 8.6% ACL + LM + MM in 4.3% ACL + MM + LM + cartilage injury (CI) in 2.9% ACL + MCL + LM in 2.9%. Conclusion: Aspiration of hemarthrosis in a traumatic knee is a good indicator for ACL injury. ACL tear is the most common knee hemarthrosis injury and often occurs as an isolated ligamentous injury, and noted that combined injuries are infrequent causes of posttraumatic hemarthrosis.