A series of 60 matched and paired patients with complete rupture of the anterior cruciate ligament (ACL) was studied; 30 were treated conservatively, even though operation was recommended, and 30 were operated on within 35 months (range 18 to 48 months) after the ACL rupture. The average age was 34 years in each group. They were assessed 39 months after arthroscopy or reconstruction. At follow up, no patient had flexion of less than 100 degrees, 13 of the reconstructed knees had an extension deficit, but in only one was this more than 10 degrees. Thirty-six percent of the reconstructed and 14% of the conservatively treated patients graded their sports activity as unlimited, while 13% of the ACL reconstructions and 21% of those treated conservatively were severely limited. The Lysholm, Cincinnati and OAK scores were significantly better in the reconstructions. The anterior drawer sign was positive in 24% of the ACL reconstructions and in 81% of the conservatively treated patients; 19% had a positive pivot shift after reconstruction compared to 75% of those treated conservatively. Even though there was a considerable number of patients with a decreased range of motion after ACL reconstruction because of the slow regime of postoperative mobilisation used, the results of operation are significantly better than after conservative treatment even when ACL reconstruction was carried out late after injury.