Background. Functional outcome measurement tools exist for individual diagnoses (eg, stroke), but no prospectively validated mobility measure is available for physical therapists’ use across the breadth of acute hospital inpatients. The modified Iowa Level of Assistance Scale (mILOA), a scale measuring assistance required to achieve functional tasks, has demonstrated functional change in inpatients with orthopedic conditions and trauma, although its psychometric properties are unknown. Objective. The aim of this study was to assess interrater reliability, known-groups validity, and responsiveness of the mILOA in acute hospital inpatients. Design. This was a cohort, measurement-focused study. Methods. Patients at a large teaching hospital in Melbourne, Australia, were recruited. One hundred fifty-two inpatients who were functionally stable across 5 clinical groups had an mILOA score calculated during 2 independent physical therapy sessions to assess interrater reliability. Known-groups validity (“ready for discharge”/“not ready for discharge”) and responsiveness also were assessed. Results. The mean age of participants in the reliability phase of the study was 62.5 years (SD = 17.7). The interrater reliability was excellent (intraclass correlation coefficient [2,1] = .975; 95% confidence interval = .965, .982), with a mean difference between scores of − .270 and limits of agreement of ± 5.64. The mILOA score displayed a mean difference between 2 known groups of 15.3 points. Responsiveness was demonstrated with a minimal detectable change of 5.8 points. Limitations. Participants were included in the study if able to give consent for themselves, thereby limiting generalizability. Construct validity was not assessed due to the lack of a gold standard. Conclusions. The mILOA has excellent interrater reliability and good known-groups validity and responsiveness to functional change across acute hospital inpatients with a variety of diagnoses. It may provide opportunities for physical therapists to collect a functional outcome measure to demonstrate the effectiveness of inpatient therapy and allow for benchmarking across institutions.