164 patients who had sustained a traumatic ACL injury causing instability and who were previously healthy in the contralateral knee were randomly assigned into 1 of 2 groups to determine the effect of ACL reconstruction on the development of OA in the operated knee. Patients were treated with either a semitendinosus tendon graft or a bone-patellar tendon-bone graft. The results of the study indicated that ACL reconstructed knees demonstrated a significant increase in the development of OA (14 years after surgical intervention) compared to contralateral knees. Osteoarthritis of the medial compartment was the most common form of OA following ACL reconstruction, and was also the most common form of OA in the contralateral knee. No difference was apparent between graft types in terms of occurrence of OA. Meniscus resection and BMI were found to be major risk factors for OA of the medial compartment. Further research is needed to compare the incidence of OA between ACL deficient knees managed operatively and nonoperatively.