The long-term effects of surgical repair and immobilization on the healing of the transected medial collateral ligament (MCL) were studied biomechanically, biochemically and histologically in a canine model. Twelve adult canines were divided into two experimental groups and studied at 48 weeks postoperatively. For Group I, the transected MCL of the left knee was not repaired, and the joint was not immobilized. For Group II, the MCL was repaired and the joint was immobilized for six weeks. The right knee of each canine was sham-operated and served as the control. Histologically, the collagen fibers were less aligned in both of the experimental groups than in the controls. Furthermore, there were minimal differences in collagen and fibroblast alignment between the groups, although poorer alignment was observed for Group I at 12 weeks. Biochemically, the levels of types I and III collagen, reducible collagen cross-links and total collagen concentration for both groups returned to normal levels. Biomechanically, Group I achieved better results than Group II in terms of varus-valgus (V-V) knee rotation and ultimate load of the femur-MCL-tibia complex (FMTC), as these values returned to the level of controls. However, the mechanical properties of the healing MCLs did not compare well with the controls; the tensile strength was only 62% and 45% of controls for Groups I and II, respectively, at 48 weeks. These results suggest that conservative treatment (i.e., no surgical intervention) with early mobilization is better than surgical treatment with immobilization for an isolated Grade III MCL injury.