In a series of 192 patients with acute knee injuries the diagnosis at arthroscopy was compared with the clinical diagnosis with or without confirmation at arthrotomy. Arthroscopy was used in 117 cases, clinical examination in 72 cases, and arthrography in 3 cases. All patients presented for diagnosis and treatment within 4 weeks of injury, and most within 1 week. The anterior cruciate ligament (ACL) was torn in 100 cases, and in 92 this was the main injury. The incidence of complete, isolated ACL tear was 1.5 per cent. ACL tear occurred in a higher proportion of cases in the arthroscopy group compared with diagnosis by other methods. An ACL tear was usually combined with injury to the medial ligament complex. Twenty-five patients showed tear of the posterior cruciate ligament (PCL), and in one-third of these there was also injury to the ACL. The medial ligaments were injured in 77 per cent of the whole series and this was the predominant lesion in 57 cases. Tear of the postero-medial capsular ligament (PMCL) was found in more than half of our patients. The ratio lateral/medial meniscus tear was 1:1.6 in the arthroscopy group and 1:27 in the group where arthroscopy was not undertaken. Arthroscopy resulted in an increase in the number of components of injury identified owing to improved diagnosis of tears of the ACL, the PMCL, the lateral meniscus, and possibly also of the PCL. There were no complication or side-effects of arthroscopy. In 93 per cent of cases primary suture was done. Reconstruction was rarely necessary if operation was undertaken within 2 weeks of injury.