To adapt the Western Ontario Rotator Cuff (WORC) index for use in Turkey and to investigate its reliability and validity; the Turkish version of the WORC was developed according to the guidelines in the literature. Seventy-two patients with rotator cuff disease were administered the questionnaire and were also evaluated by using the University of California Los Angeles (UCLA) shoulder rating scale, Constant score, and Short Form (SF)-36 to test validity. The WORC questionnaire was repeated in 35 patients after a mean interval of 2.9 days (range 2-7 days) to evaluate test-retest reliability. Cronbach's alpha was calculated as 0.92 for the total questionnaire. The intraclass correlation coefficients were very high and ranged between 0.96 and 0.98 for each section. There was a significant negative correlation between the Turkish version of WORC and UCLA (r = -0.598, P < 0.01), Constant score (r = -0.630, P < 0.01), and all subscales of SF-36 (P < 0.01). The Turkish version of the WORC index is a reliable and valid instrument for use in clinical trials in patients with rotator cuff disorders.