Objective: To test the Cumberland Ankle Instability Tool (CAIT), a 9-item 30-point scale, for measuring severity of functional ankle instability. Design: Cross-sectional study. Setting: General community. Participants: Volunteer sample of 236 subjects. Interventions: Not applicable. Main Outcome Measures: Concurrent validity by comparison with the Lower Extremity Functional Scale (LEFS) and a visual analog scale (VAS) of global perception of ankle instability by using the Spearman p. Construct validity and internal reliability with Rasch analysis using goodness-of-fit statistics for items and subjects, separation of subjects, correlation of items to the total scale, and a Cronbach a equivalent. Discrimination score for functional ankle instability by maximizing the Youden index and tested for sensitivity and specificity. Test-retest reliability by intraclass correlation coefficient, model 2,1 (ICC2,1). Results: There were significant correlations between the CAIT and LEFS (rho=.50, P <.01) and VAS (rho=.76, P0.5; item reliability index, .99). The threshold CAIT score was 27.5 (Youden index, 68.1); sensitivity was 82.9% and specificity was 74.7%. Test-retest reliability was excellent (ICC2.1 = .96). Conclusions: CAIT is a simple, valid, and reliable tool to measure severity of functional ankle instability.