A comparative study of the diagnostic accuracy between the Lachman test and the anterior drawer test was performed by examining 144 knees with ruptured anterior cruciate ligaments. The diagnosis of anterior cruciate ligament rupture is often difficult to establish, especially in recent injuries with acute haemarthrosis. The diagnostic accuracy of the Lachman test in recent ruptures when the patient is examined without general anaesthetic is superior to that of the anterior drawer test, while in chronic cases with third-degree instability the two tests are equally reliable.