STUDY DESIGN: Clinical measurement study. OBJECTIVES: To translate and cross-culturally adapt the Western Ontario Shoulder Instability Index (WOSI) into Italian, and to evaluate its measurement properties in patients with shoulder instability secondary to a first-time traumatic anterior dislocation. BACKGROUND: The WOSI was developed for English-speaking patients. To date, no Italian version of the WOSI exists. METHODS: The WOSI was cross-culturally adapted to Italian according to established guidelines. Sixty-four (16 women, 48 men) patients with unilateral shoulder anterior instability were prospectively recruited for the purposes of this study. Internal consistency, test-retest reliability, construct validity, and responsiveness of the WOSI were evaluated. RESULTS: The Italian version of the WOSI showed a high degree of internal consistency, with a Cronbach alpha of .93 (95% confidence interval [CI]: 0.91, 0.96). The test-retest reliability was high for both short-term (3 days, 64 patients) and medium-term (14 weeks, 20 patients) test-retest, with intraclass correlation coefficients of 0.95 (95% CI: 0.90, 0.97) and 0.92 (95% CI: 0.89, 0.95), respectively. The WOSI was more closely correlated to the Disabilities of the Arm, Shoulder and Hand questionnaire than to the Medical Outcomes Study 36-Item Short-Form Health Survey (r = 0.794 and 0.113, respectively). The receiver-operating-characteristic curve analysis revealed that the WOSI was more responsive than the Disabilities of the Arm, Shoulder and Hand questionnaire (P = .03), with an area under the curve of 0.90 (95% CI: 0.78, 0.97) for the WOSI and 0.76 (95% CI: 0.61, 0.88) for the Disabilities of the Arm, Shoulder and Hand questionnaire. CONCLUSION: The Italian version of the WOSI is a valid, reliable, and responsive tool that can be used to measure function in Italian-speaking patients with shoulder instability due to a first-time traumatic anterior dislocation.