It has been suggested that self-efficacy belief is of major importance for rehabilitation outcome after sports-related injuries. No instruments are, however, available to evaluate perceived self-efficacy for prognostic and outcome expectations in patients with an anterior cruciate ligament (ACL) injury. Perceived self-efficacy is defined as a judgment of one's potential ability to carry out a task, rather than a measure of whether or not one actually can or does perform the task. The purpose of this study was to develop a reliable and valid instrument for measuring perceived self-efficacy in patients with an ACL injury. A total of 210 male and female patients with an ACL injury were included in this study. The items were generated by health professionals with long clinical experience of patients with an ACL injury and by discussions with patients. After item analysis and item reduction, based on the results from 88 patients, the final 22-item version of the Knee Self-Efficacy Scale (K-SES) was evaluated in 18 patients for test-retest reliability and in 104 patients for internal consistency and validity. The K-SES was compared with the Multidimensional Health Locus of Control (MHLC), Coping Strategies Questionnaire (CSQ), SF-36 and Knee Injury and Osteoarthritis Outcome Score (KOOS) instruments. A factor analysis was also performed on the K-SES. The test-retest revealed a correlation of r(s)=0.73 between test-days and an intraclass correlation coefficient of 0.75. No significant difference between test-days was found. The internal consistency was 0.94, as calculated with Cronbach's alpha. There were low correlations between the K-SES and MHLC and the K-SES and CSQ, respectively. A strong correlation was found between the K-SES and physical functioning, as measured by the SF-36 (r(s)=0.8). All the sub-scales in the KOOS correlated moderately to strongly (r(s)=0.4-0.7) to the K-SES. The factor analysis produced two factors of importance. Factor one was related to how patients perceived their present physical performance/function, while factor two was related to how patients perceived the future physical performance/prognosis of their knee. Good reliability and good face, content, construct and convergent validity were demonstrated for this new instrument (K-SES) for measuring perceived self-efficacy in patients with an ACL injury. The K-SES is recommended for studies designed to evaluate prognostic and outcome expectations of perceived self-efficacy in patients with an ACL-insufficient knee.