PURPOSE: The purpose of this study was to evaluate content validity, test-retest reliability, internal consistency, construct validity, responsiveness and floor/ceiling effects of Forgotten Joint Score (FJS) for assessing functions in activities of daily living (ADL) following anterior cruciate ligament reconstruction (ACLR) and perform an item reduction of FJS. METHODS: Swedish-speaking ACLR patients in one surgical clinic were eligible. Content validity was evaluated through patient responses and patient and expert clinician relevance ratings, omitting items with low relevance. Principal component factor analysis, Cronbach's , paired test, correlations between FJS and Knee Injury and Osteoarthritis Outcome Score (KOOS), Cohen's effect sizes (ESs) and standardized response mean (SRM) were used to evaluate internal consistency, test-retest reliability, construct validity and responsiveness. Floor/ceiling effects were calculated. FJS was expected to reveal one dominant factor, a Cronbach's between 0.70 and 0.95, correlations >0.50 to all KOOS subscores, highest for ADL, moderate ES and SRM, floor/ceiling effects <15%. RESULTS: One hundred and seventy-six participants (103 for assessing internal consistency, construct validity, responsiveness and floor/ceiling effects; 73 for assessing test-retest reliability and content validity) were included. Item reduction yielded a nine-item FJS (FJS-9). FJS-12 and FJS-9 demonstrated sufficient content validity (95% confidence interval [CI], 2.5-3.1, respectively, 2.9-3.3). FJS-9 was found unidimensional, and FJS-12 was multidimensional. Cronbach's was 0.94 for FJS, and the intraclass correlation coefficient > 0.90. FJS showed significant correlations >0.65 to all KOOS subscales, moderate ES and SRM > 0.50. No floor/ceiling effects were found. CONCLUSION: FJS-9 demonstrated adequate validity for the evaluation of joint awareness in ACLR patients and can be a valuable tool to assess ADL and joint awareness. LEVEL OF EIDENCE: Level III.