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Objective: To translate into Arabic and validate the Western Ontario and McMaster Universities (WOMAC) index., Method: Arabic translation was obtained with use of the forward and backward translation method. Adaptations were made after a pilot study. Patients with symptomatic knee OA fulfilling the revised criteria of the American College of Rheumatology were included. Impairment outcome measures (pain as measured on a visual analog scale, the maximum distance walked, Kellgren's radiological score), Lequesne index score and Beck depression scale score were recorded. Each item was analyzed. Test-retest reliability was assessed with use of the intra-class correlation coefficient (ICC) and the Bland and Altman method. Construct validity was investigated with use of Spearman's rank correlation coefficient, and a factor analysis was performed., Results: One hundred and three patients were included in the study. Eight questions of the WOMAC physical function subscale (PF) had insufficient psychometric properties and were excluded. Although test-retest reliability of the questionnaire was good (0.84, 0.84, and 0.92 for pain, stiffness, and modified PF subscales respectively), construct validity could not be demonstrated. Factor analysis of the modified form of the WOMAC extracted four factors, which differed from the a priori triple stratification. However, factor analysis of the modified PF subscales extracted two factors, which accounted for 68.4% of the total variance and could be clinically characterized (disability during activities requiring knee flexion within the first 90 degrees and activities requiring knee flexion over more than 90 degrees )., Conclusion: We translated and adapted the WOMAC index into Arabic to suit Tunisian people. The translated questionnaire is reliable but not valid in its original form. We propose the use of a modified version of PF subscale of the WOMAC, although the psychometric properties of this instrument must be examined in a larger population.