The purpose of this study was to develop a valid, reliable, and responsive disease-specific quality of life measurement tool for patients with shoulder instability. Development included 1) identification of a specific patient population; 2) generation of issues specific to the "disease" ("items") from reviewing the literature, interviewing health caregivers, and interviewing patients representing all demographics, disease type and severity, and treatments; 3) item reduction using patient-generated frequency-importance products and correlation matrices; and 4) pretesting the prototype instrument on 2 groups of 10 patients. The final instrument, the Western Ontario Shoulder Instability Index, has 21 items representing 4 domains. The instrument attributes (validity, reliability, and responsiveness) were evaluated. Construct validation demonstrated that this index correlated predictably with other measures. Reliability was very high at 2 weeks and 3 months, and the index was more responsive (sensitive to change) than five other shoulder measurement tools (the Disabilities of the Arm, Shoulder and Hand scale; The American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form; the UCLA Shoulder Rating Scale; the Constant Score; and the Rowe Rating Scale), a global health instrument (the SF12), and range of motion. Since the patient's perception of changes in health status is the most important indicator of the success of a treatment, we suggest that this measurement tool be used as the primary outcome measure to evaluate treatments in this patient population, although it can also be used for monitoring patients' progress in clinical practice.