The purpose of this study was to examine prognostic validity, concurrent validity and responsiveness of ankle function score (AFS) for patients with acute ankle injury. In a prospective cohort study, AFS was compared with Olerud and Molander ankle score (OMAS), patient-specific complaints (PSC), and global perceived effect. Sensitivity and specificity were calculated to estimate prognostic validity; correlation was calculated for concurrent validity. Effect Size, Standardized Response Mean, and Responsiveness Ratio were calculated to estimate responsiveness. The sensitivity of AFS was 76% and the specificity was 57%. Correlation between AFS and OMAS at baseline, end of treatment and difference was 0.82, 0.70 and 0.79, respectively. Outcomes for responsiveness of AFS were comparable to outcomes for OMAS and PSC. This study found limited evidence for the AFS as a prognostic and evaluative instrument. The AFS is a simple instrument based on basic functional outcomes (pain, stability, weight bearing, swelling and gait) and may be used in addition to subjective clinical judgment as a prognostic and evaluative tool for recovery after acute ankle injury.