One hundred thirteen consecutive athletes, who had sustained significant acute trauma to the knee wit; immediate disability and the early onset of hemarthrosis but who did not have demonstrable clinical laxity, were examined under anesthesia and had arthroscopy within 3 weeks of injury (the majority within 10 days). Lesions of surgical significance were found in 102 of the 113 cases (90%). Anterior cruciate ligament tears were present in 81 (72%), of which two-thirds also had associated meniscus lesions. Major meniscus tears with no associated cruciate lesions were found in 17 cases (15%), osteochondral fractures in 7 cases (6%), posterior cruciate ligament tears in 3 cases (3%), and no internal derangement in 5 cases (4%). Acute knee injuries with hemarthrosis, rather than being a contraindication to arthroscopy, are in fact one of the best indications for use of this procedure.