A total of 220 consecutive young adults with a traumatic effusion of the knee joint, seen initially in the accident and emergency department, have been reviewed in a weekly orthopaedic acute knee clinic. Of the patients, 80 per cent were seen within 3 days of the injury, and all patients were seen within 8 days of the injury. There were 62 patients (28 per cent) with damage to the anterior cruciate ligament (ACL), of whom 37 patients (17 per cent) had acute complete ACL tears. There were 92 haemarthroses in this series, in which there was a high incidence of ACL damage. The Lachman test was used in this study and identified 73 per cent of the acute complete ACL tears preoperatively and all the chronic ACL injuries. Acute ACL injuries can be diagnose early provided adequate resources are available to provide clinic and theatre facilities. Early diagnosis enables the patients to be given clear advice on future job and sports prospects and allows selection of patients most likely to benefit from augmented repair of the ligament. Associated meniscal lesions can also be identified and treated at an early stage.