To help elucidate the longterm outcome in the unstable knees of dogs that have undergone anterior cruciate ligament (ACL) transection, conventional radiography, gait analysis and magnetic resonance imaging (MRI) were performed serially on 3 dogs over a 45-month period after ACL transection. Gait analysis showed that vertical forces generated by the cruciate deficient leg were smaller than those produced by the contralateral leg, but the decrease in loading was not progressive and clinical findings did not suggest that the dogs developed pain in the unstable limb. Despite progressive osteophytosis and subchondral sclerosis, as seen on plain radiographs, MRI demonstrated that articular cartilage in the unstable knee was thicker than that in the contralateral knee 36 months after ACL transection. Nine months later, however, striking focal loss of articular cartilage was seen in the unstable knee of each dog. Our study, which provides the first longterm observations of the changes in the unstable knee after ACL transection, shows that they represent progressive osteoarthritis and emphasizes that a prolonged phase of cartilage hypertrophy may precede the stage of cartilage breakdown.