The knee injury and Osteoarthritis Outcome Score (KOOS), based on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), is widely used to evaluate subjective outcome in anterior cruciate ligament (ACL) reconstructed patients. However, the validity of KOOS has not been assessed using Rasch analysis. The objective of this study was to evaluate the viability of KOOS as an outcome measure for ACL reconstruction using the partial credit Rasch model. Rasch analysis was applied to 200 KOOS questionnaires completed by patients consecutively tested 20 weeks after ACL reconstruction and subsequent rehabilitation. Rasch analysis showed that of the five proposed subscales in KOOS, only knee-related quality of life (QoL) and sport and recreational related function (Sport/Rec) fulfilled the criteria of a unidimensional measurement scale when applied to these patients. The three subdomains in KOOS extracted from WOMAC did not fulfill these criteria. While the content of KOOS appears to be relevant for knee patients, the psychometric measurement properties of KOOS are insufficient for use on patients 20 weeks subsequent to ACL reconstruction. A new knee measure targeted for these patients could be developed based on the content of KOOS. This study demonstrates that knee measurement instruments constructed for a specific condition cannot necessarily be used on patients with other similar conditions.