PURPOSE: To translate and adapt the Norwich Patellar Instability (NPI) score into Norwegian, and second, to examine the psychometric properties of the Norwegian version (NPI-No). METHODS: NPI was translated according to international guidelines. A cohort of 107 patients surgically treated for recurrent patellofemoral instability completed NPI-No, related questionnaires and functional tests prior to and six months post-surgery. Validity (face, content and construct validity), internal consistency (Cronbach's alpha [ RESULTS: NPI-No had good face and content validity. Internal consistency was satisfactory ( CONCLUSION: The NPI-No is valid for assessment of self-perceived patellar instability before and after surgery in Norwegian patients. However, reproducibility was found to be only moderate. This study adds further knowledge about the measurement properties of the NPI. LEVEL OF EVIDENCE: Level II.