BACKGROUND: In people with anterior cruciate ligament (ACL) injury, high self-efficacy facilitates recovery, indicated by improved muscle function, reduced knee symptoms and increased physical activity. Impact of treatment on future self-efficacy is however not well investigated. The aims of the study were to 1) investigate knee-related self-efficacy 6 years after acute ACL injury in patients treated with exercise therapy alone or in combination with either early or the option of delayed ACL reconstruction (ACLR), and 2) to investigate associations between single-leg physical performance at various time points after ACL injury and knee self-efficacy at 6 years after injury.METHODS: Participants (n =?121) originated from the KANON-study (ISRCTN84752559), a treatment RCT including active adults with acute ACL injury treated with structured exercise therapy combined with early or the option of delayed ACLR. In this ancillary study, participants with knee self-efficacy data at 6 years (n =?89) were analyzed as treated; exercise therapy alone (n =?20), exercise therapy plus early ACLR (n =?46), and exercise therapy plus delayed ACLR (n =?23). The participants performed physical performance tests (hop, strength and balance) at the end the of exercise therapy (mean 10 (SD 6) months), and at 5 years, and rated their knee self-efficacy using Knee Self-Efficacy Scale (K-SES) questionnaire (0 to 10, worst to best) at 6 years.RESULTS: Median K-SES score for the total group (n =?89) was 7.8 (IQR 5.9-9.0). There were no differences between treatment groups in K-SES scores at 6 years nor in physical performance at any time point (p =?0.097). Worse knee flexion strength LSI (rsp =?0.341, p =?0.042) at the end of the exercise therapy, and worse LSI for single-leg hop test (rsp =?0.310, p =?0.005) at the end of the exercise therapy and at 5 years, correlated moderately with worse knee-related self-efficacy at 6 years. Low associations were observed between the remaining physical performance tests and K-SES scores (rsp =?0.265, p =?0.045).CONCLUSION: Knee-related self-efficacy at 6 years after ACL injury did not differ between those treated with ACLR, performed early or as a delayed procedure, or exercise therapy alone. Good physical performance at the end of the exercise therapy, and at 5 years, appears to have a positive, yet small, impact on future knee-related self-efficacy.