OBJECTIVE:To validate the WOMAC Osteoarthritis Index for use in Sweden. METHODS:Test-retest reliability, internal consistency, validity, and responsiveness was determined in 52 patients (mean age 48 (20-69)) with arthroscopically assessed cartilage damage of the tibio-femoral knee joint. RESULTS:All WOMAC scales were internally consistent with Cronbach's alpha coefficients of 0.83, 0.87, and 0.96 pre-operatively. Test-retest reliability was satisfactory with intraclass correlation coefficients of 0.74, 0.58, and 0.92. As hypothesized worse post-operative but not pre-operative outcomes were associated with radiographic OA. In comparison with the SF-36 the expected correlations were found when comparing items measuring similar and dissimilar constructs, supporting the concepts of convergent and divergent construct validity. Three months after arthroscopy significant mean improvement was seen in all WOMAC scales (p<0.0004). CONCLUSION:The Swedish version of WOMAC is a reliable, valid, and responsive instrument with metric properties in agreement with the original widely used version.