Arthrosis following rupture of the anterior cruciate ligament has been analysed in two series. The first series was derived from a review of 150 cases of reconstruction of the anterior cruciate ligament with a follow-up of 3 years or more. Arthrosis was seen to have developed in 13.3 %. The second series was concerned with 64 cases of unilateral arthrosis treated by upper tibial valgus osteotomy in whom there had been a previous rupture of the anterior cruciate ligament. The ‘tolerance interval’ – that is the time between the original ligamentous injury and the time of osteotomy – for the development of arthrosis was very variable, ranging in the natural-history cases from 10 to 50 years, with a mean of 35 years. It is important to recognise the radiological signs of the onset of arthrosis. These are osteophytosis of the intercondylar notch, osteophyte formation at the posterior part of the medial tibial plateau, and, in particular, narrowing of the medial joint line with posterior subluxation of the medial femoral condyle, well seen in lateral radiographs whilst standing on one lower limb. Early arthroses, appearing after 10 years, may occur as a ‘natural arthrosis’, but it develops much more frequently after surgical treatment that had failed to correct anterior laxity and particularly when it had been performed on knees that were already pre-arthrotic. The main factor in arthrosis is anterior laxity measured radiologically by an ‘active Lachman’ radiograph. Removal of the medial meniscus which in itself, is liable to produce arthrosis, is even more harmful in anterior cruciate laxity since it doubles the degree of anterior subluxation of the tibia seen on unilateral weight-bearing. The development of varus deformity, which characterises progressive arthrosis, has its origin in wear of the posterior part of the medial tibial plateau caused by anterior cruciate laxity. Other factors play an important part such as associated lateral laxity, constitutional genu varum and weakness of the hamstring muscles, which oppose the subluxating action of the quadriceps.