Context: The underlying mechanism in 27% of ankle sprains is a fall while navigating stairs. Therefore, the step-down test (SDT) may be useful to investigate dynamic postural stability deficits in individuals with chronic ankle instability (CAI). Objective: To investigate the test–retest reliability and validity of the forward and lateral SDT protocol between individuals with CAI and uninjured controls. Design: Test–retest study. Setting: University hospital. Participants: A total of 46 individuals, 23 with CAI and 23 uninjured controls. Main Outcome Measures: Time to stabilization of the forward and lateral SDT. Results: The absolute reliability (SEM = 0.04–0.12 s; SDD = 0.11–0.33 s) of the SDT protocol was acceptable, whereas the relative reliability (ICC3, k = 0.12–0.63) and discriminant validity (P = .42–.99; AUC = 0.50–0.57) were not. Conclusions: The SDT appears to not be challenging enough to detect dynamic postural stability differences between individuals with and without CAI. However, the SDT may be capable of measuring change over time based on its good absolute reliability.