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Purpose: A limited understanding of how functional ankle instability (FAI) affects dynamic postural stability exists because of a lack of reliable and valid measures. Therefore, the purpose of this investigation was to determine whether a new reliable index for dynamic postural stability could differentiate between those with stable ankles and those with FAI. Methods: Data were collected on 108 subjects (54 subjects with stable ankles (STABLE group); 54 subjects with functionally unstable ankles (FAI group)). Subjects performed a single-leg-hop stabilization maneuver in which they stood 70 cm from the center of a force plate, jumped off both legs, touched a designated marker placed at a height equivalent to 50% of their maximum vertical leap, and landed on a single leg. The dynamic postural stability index and directional stability indices (medial/lateral, anterior/posterior, and vertical) were calculated. The raw and normalized (to energy dissipated) indices were compared between groups. Results: Significant differences were noted for the anterior/posterior stability index (FAI = 0.36 ± 0.09, STABLE = 0.30 ± 0.06). Similar results were seen for the vertical stability index (FAI = 0.73 ± 0.17, STABLE = 0.61 ± 0.13), the normalized dynamic postural stability index (FAI = 0.85 ± 0.17, STABLE = 0.73 ± 0.12), the normalized vertical stability index (FAI = 0.007 ± 0.004, STABLE = 0.005 ± 0.001), and the dynamic postural stability index (FAI = 0.008 ± 0.003, STABLE = 0.006 ± 0.001). Conclusions: These results indicate that the dynamic postural stability index is a sensitive measure of dynamic postural stability and is capable of detecting differences between individuals with stable ankles and individuals with functionally unstable ankles.