In this study, 400 clinically anterior cruciate ligament (ACL) deficient knees were arthroscoped and studied prospectively in the period January 1986 to April 1992. An ACL tear was always confirmed, and 41 per cent of these patients did not have an associated meniscal tear. In 30.25 per cent the lateral meniscus was torn; in 21.25 per cent the ACL tear was associated with a medial meniscus tear, and in the remaining 7 per cent both menisci were torn. The most frequently associated meniscal injury was the bucket handle tear of the medial meniscus (9 per cent), followed by the posterior horn tear of the lateral meniscus, which showed the same frequency as the ragged (or degenerated) tear of the lateral meniscus (6 per cent). The horizontal tear of the posterior part of the lateral meniscus showed a prevalence of 4.3 per cent. This picture is probably dependent on a secondary referral nature of the centre surveyed, in which the average time between injury and arthroscopy was 23.3 months.