<h3>Research Objectives</h3> To the assess construct validity and reproducibility of walking-adaptability assessment on an instrumented treadmill (the C-Mill) in polio survivors. <h3>Design</h3> Observational study with two repeated walking-adaptability measurements (target stepping, with 0%, 20% and 30% target position variance, and anticipatory and reactive obstacle avoidance), separated by 2-3 weeks. <h3>Setting</h3> Outpatient clinic of a university hospital. <h3>Participants</h3> Polio survivors (n=46) who fell in the previous year and/or reported fear of falling. <h3>Interventions</h3> Not applicable. <h3>Main Outcome Measures</h3> We examined 1) face validity by determining Group (for subgroups stratified for fall frequency, fear of falling and leg-muscle weakness) and Condition effects (for difficulty level) on walking-adaptability outcomes, 2) construct validity by correlating walking-adaptability and balance outcomes, and 3) content validity by establishing possible ceiling effects. We determined whether face-validity findings were reproducible over test occasions and calculated Intraclass Correlation Coefficients (ICC) and the 95% Limits of Agreement (LoA) for walking-adaptability outcomes. <h3>Results</h3> Walking-adaptability outcomes differed in to-be-expected directions for subgroups stratified for fall frequency and leg-muscle weakness and for difficulty levels, all reproducible over test occasions. Correlations between walking-adaptability and balance outcomes were mainly low (r < 0.587). Ceiling effects were present for anticipatory obstacle-avoidance and balance outcomes, but not for reactive obstacle avoidance. ICCs [95% confidence intervals] were good for the challenging 20% (0.80[0.67-0.88]) and 30% target-stepping (0.74[0.57-0.85]) and reactive obstacle-avoidance (0.76[0.59-0.86]) conditions, but not for 0% target-stepping and anticipatory obstacle-voidance (ICC < 0.62) conditions. Likewise, the narrowest LoA were observed for the 20% and 30% target-stepping conditions. <h3>Conclusions</h3> We proved face, construct and content validity of C-Mill walking-adaptability assessment in polio survivors with a history and/or fear of falling. Adding walking-adaptability assessment, particularly the more challenging tests given their superior reproducibility, to currently used clinical tests could improve fall-risk evaluation in this population. <h3>Author(s) Disclosures</h3> None.