Long-term changes in the three-dimensional external loading, hind limb kinematics and knee stability were assessed in an anterior cruciate ligament (ACL)-transected cat model of osteoarthritis (OA). Seven skeletally mature cats (mean mass 4.6+/-1.4 kg) were studied before ACL transection (ACLT) and at 1 and 3 weeks, and at 3, 6, 9 and 12 months following ACLT. One week following ACLT, significant changes from the normal locomotion pattern were observed: peak vertical and anterior posterior ground reaction forces were decreased, particularly the peak posterior forces in the early phase of stance. Furthermore, knee angles were reduced by about 15 degrees throughout the whole gait cycle, while ankle and hip angles were reduced at paw off in the experimental compared to the contralateral hind limbs. Ground reaction forces and hind limb kinematics recovered to near pre-surgical patterns over the one year period assessed. ACLT was also associated with an increased knee instability which improved over time. X-rays suggested that there was a continued degeneration in the experimental knee over the one year period; there was osteophyte formation at the joint margins and an increase in cartilage thickness throughout the joint. It was speculated that the more flexed knee angles and the reduced anterior-posterior ground reaction forces in the ACL-transected compared to the intact hind limb represent an adaptive strategy aimed at avoiding excessive anterior displacement of the tibia in the early phase of stance. The recovery of the locomotion pattern with time might be related to the corresponding improvement of knee stability.