PURPOSE: To develop and psychometrically test a Brazilian-Portuguese version of the Cumberland Ankle Instability Tool (CAIT), the only questionnaire that provides a numeric measure for functional ankle instability. METHODS: The CAIT was translated and adapted into Brazilian-Portuguese according to the Guidelines for the process of cross-cultural adaptation of self report measures. The Brazilian-Portuguese version of the CAIT was tested for internal consistency, test-retest reliability, ceiling and floor effects and responsiveness in 131 participants. Participants were recruited from the general community in Brazil (N = 101, community group) and from those seeking treatment for an ankle sprain from 2 clinics in Brazil (N = 30, treatment group). RESULTS: The Brazilian-Portuguese version of the CAIT had high internal consistency (Cronbach alpha = 0.86 for right ankles and 0.88 for left ankles), reliability (ICC = 0.95, 95% CI 0.93-0.97); and good responsiveness (ES = 0.75, 95% CI 0.49-1.00). No ceiling or floor effects were observed. CONCLUSIONS: The Brazilian-Portuguese version of the CAIT is as reliable as the English version of the questionnaire, has high internal consistency and good responsiveness. It thus provides the first tool that can be used to assess functional ankle instability by clinicians and researchers working among Brazilian-Portuguese speakers.