Background: One of the most disturbing long term sequelae of an anterior cruciate ligament (ACL) injury is the development of early onset osteoarthritis (OA) in younger people. Controversy exists regarding whether surgical or non-surgical approaches to treatment are associated with the development of OA. An earlier study by the authors has shown that no statistically significant difference exists in OA between patients treated surgically or conservatively at 12 years post injury. However when the surgical groups were sub-grouped into patellar-tendon (PT) and semitendinosus/gracilis (STG) grafting groups, significantly less OA was found in the STG group. It is possible that sub-grouping conservatively treated ACL patients may provide similar insights regarding the development of OA. Purpose: This study aimed to compare the incidence of tibiofemoral and patellofemoral OA in 3 sub-groups of conservatively treated ACL patients namely, an unstable (non-copers), an equivocally stable (potential copers or adaptors) and a stable (copers) sub-group, at 12 years post injury. Methods: 52 conservatively managed ACL injured patients, were recruited from a cohort of 297 ACL injured patients previously referred to one physiotherapy clinic. These comprised 32 males and 20 females, mean aged 40 years. Patients were included in the study if they had a ruptured ACL on MRI, arthroscopy or clinical examination by an orthopaedic surgeon and had no reconstructive surgery. 34 patients with meniscal injuries were included. Assessment included subjective, instrumented (KT 1000) and clinical stability testing. According to their levels of stability the patients were sub-grouped into stable, equivocally stable and unstable sub-groups. Bilateral knee radiographs were taken and graded by an independent radiologist blind to the aims of the study. Tibiofemoral and patellofemoral OA was graded as absent, mild, moderate or severe based on the Kellgren model. An ANOVA was conducted to compare the incidence and severity of OA between the 3 sub-groups. Results: 15 patients had unstable ACL-deficient knees, 23 had equivocally stable knees and 14 had stable ACL-deficient knees. Overall 63% of patients developed tibiofemoral and 44% developed patellofemoral OA. 80% of the unstable group, 63% of the equivocal group and 50% of the stable group developed tibiofemoral OA. ANOVA studies revealed that there was a statistically significant difference in the incidence of tibiofemoral OA between the stable and unstable group (p = 0.017). No differences were found for the development of patellofemoral OA. Conclusion(s): Very few long-term follow-up studies of conservatively managed ACL cohorts exist. This study has shown that among ACL-injured patients at 12 years post injury, there exists a group that remains stable and is able to return to the same sporting level as prior to injury. This stable group developed significantly less tibiofemoral OA in the long term compared to less stable groups. Implications: This study has highlighted the importance of detailed assessment of ACL injured patients into stability groups prior to making decisions re surgery. It is possible that the more stable knee treated conservatively may develop less OA than if treated surgically.