The ICU Mobility Scale (IMS) is a measure of mobility milestones in critically ill patients.This study aimed to determine the validity and responsiveness of the IMS from a prospective cohort study of adults admitted to the intensive care unit (ICU).Construct and predictive validity were assessed by comparing IMS values at ICU discharge in 192 patients to other variables using Spearman rank correlation coefficient, Mann-Whitney U tests, and logistic regression. Responsiveness was assessed using change over time, effect size, floor and ceiling effects, and percentage of patients showing change.The IMS at ICU discharge demonstrated a moderate correlation with muscle strength (r = 0.64, P < 0.001). There was a significant difference between the IMS at ICU discharge in patients with ICU-acquired weakness (median, 4.0; interquartile range, 3.0-5.0) compared with patients without (median, 8.0; interquartile range, 5.0-8.0; P < 0.001). Increasing IMS values at ICU discharge were associated with survival to 90 days (odds ratio [OR], 1.38; 95% confidence interval [CI], 1.14-1.66) and discharge home (OR, 1.16; 95% CI, 1.02-1.32) but not with return to work at 6 months (OR, 1.09; 95% CI, 0.92-1.28). The IMS was responsive with a significant change from study enrollment to ICU discharge (d = 0.8, P < 0.001), with IMS values increasing in 86% of survivors during ICU admission. No substantial floor (14% scored 0) or ceiling (4% scored 10) effects were present at ICU discharge.Our findings support the validity and responsiveness of the IMS as a measure of mobility in the ICU.