The knee self-efficacy scale (K-SES) has been shown to have good reliability, validity and responsiveness during rehabilitation for patients' perceived self-efficacy of knee function. Determinants of self-efficacy of knee function 1 year after ACL reconstruction have been found to be the patients' internal locus of control and knee symptoms in sports and recreation. The predictive ability of perceived self-efficacy of knee function measured by the K-SES has not been studied in terms of patient outcome after an ACL reconstruction. The purpose of this study was to explore the potential for pre-operative self-efficacy of knee function measured by the K-SES to predict patient outcome in terms of physical activity, knee symptoms and muscle function 1 year after an ACL reconstruction. Thirty-eight patients were evaluated for outcome in terms of physical activity with the Tegner activity scale and the physical activity scale (PAS), knee symptoms with the Lysholm knee scoring scale and the knee injury and osteoarthritis outcome score (KOOS) and knee function with ability tests for muscle function 1 year after ACL reconstruction. Multiple regression and logistic multiple regression analysis were used to evaluate the K-SES as a possible predictor of outcome. The patients' present perceived self-efficacy of knee function (K-SES(Present)) pre-operatively was a significant predictor (P = 0.016) of the patients returning to their intensity and frequency of physical activity (PAS) 1 year after ACL reconstruction, when adjusted for age, gender and pre-injury physical activity level (Tegner(Pre-injury)) (odds ratio = 2.1). The patients' perceived future self-efficacy of knee function (K-SES(Future)) pre-operatively was a significant predictor (P = 0.045) of their self-rated knee function in sports/recreational activities (KOOS(Sports/recreation)) at the 1-year follow-up, when adjusted for age, gender and Tegner(Pre-injury )(R (2 )=0.25). The pre-operative K-SES(Future) was also a significant predictor (P = 0.023) of the patients' knee-related quality of life (KOOS(Qol)), at the 1-year follow-up, when adjusted for age, gender and Tegner(Pre-injury) (R (2 )=0.23). The pre-operative K-SES(Future) was furthermore a significant predictor of an acceptable outcome 1 year after surgery, on the Lysholm knee scoring scale (P = 0.003, odds ratio = 1.7), as well as on KOOS(Sports/recreation) (P = 0.002, odds ratio = 1.6) and knee-related quality of life (KOOS(Qol)) (P = 0.037, odds ratio = 1.4), when adjusted for age, gender and Tegner(Pre-injury). The pre-operative K-SES(Future) was also a significant predictor (P = 0.04) of an acceptable outcome 1 year after surgery, on the one-leg hop for distance (odds ratio = 2.2), when adjusted for age, gender and Tegner(Pre-injury). In conclusion, this study indicates that patients' perceived self-efficacy of knee function pre-operatively is of predictive value for their return to acceptable levels of physical activity, symptoms and muscle function 1 year after ACL reconstruction.