Acute hemarthrosis of the knee usually results from a significant injury. Preoperative assessment, examination under anesthesia, and arthroscopic examination were conducted on patients with a total of 132 acutely injured knees associated with hemarthrosis and minimal clinical laxity to determine the presence and extent of injury to ligamentous and/or intra-articular structures. Partial or complete tear of the anterior cruciate ligament was found in 101 (77%) of the knees. Peripheral meniscal tears were responsible for the hemarthrosis in 17 cases (13%), and osteochondral fractures were found in 11 cases (8%). Injury to the anterior cruciate ligament was frequently associated with concomitant injury to other structures: meniscal tears (61%), ligament/capsular injury (40%), and hyaline chondral damage (16%). Patients' responses to preoperative anterior drawer, pivot shift, and Lachman tests were within normal limits in 18%, 29%, and 73% of cases, respectively. Patients under anesthesia responded to the anterior drawer and pivot shift tests normally 50% and 74% of the time. The Lachman test, performed under anesthesia, was 98% accurate in predicting anterior cruciate injury. Experience gained in evaluating posttraumatic hemarthrosis of the knee suggests clinical examination alone may not demonstrate the severity of many of these injuries. Arthroscopy, although not necessary to determine competence of the anterior cruciate ligament, is invaluable in determining the existence of other injuries with or without an associated anterior cruciate ligament tear.