The purpose of this study was to evaluate the long-term clinical outcome of operative versus nonoperative treatment of anterior cruciate ligament (ACL) deficiency and to define its relationship with sports activity. Forty-six patients (37 male, 9 female, mean age, 33.6 +/- 8.0 years) who underwent open ACL reconstruction using patellar tendon autograft and 25 patients (18 male, 7 female, mean age, 32.3 +/- 9.9 years) who were treated nonoperatively were evaluated by the same two examiners at 5 - 7 and 10 - 13 years following injury. The evaluations included objective and subjective scoring (Lysholm, OAK, IKDC), instrumented testing (Cybex, KT 1000), radiographic evaluation, and assessments of sports activity, with respect to type, frequency and associated symptoms. According to Lysholm, OAK and IKDC scores, the operative group performed significantly (p < 0.05) better and was able to maintain increased involvement in sports, although both groups participated less over time. However, risks for degenerative joint changes were similar for both the operative and the nonoperative group. A significant (p < 0.05) correlation between participation in high-risk pivoting sports, such as soccer or basketball and osteoarthritic changes could be found for the nonoperative group, only. Sports activity represents an important variable not only affecting the outcome, but also influencing treatment decisions following ACL injuries.