INTRODUCTION: To assess self-reported physical functioning in patients with sporadic inclusion body myositis (sIBM), the sIBM Physical Functioning Assessment (sIFA) was developed. This research establishes the validity, reliability, and responsiveness of the sIFA in patients with sIBM. METHODS: Data from 3 small, noninterventional, observational studies were analyzed. Several measures of physical function were included to assess validity. Reliability (Cronbach alpha, test-retest intraclass correlations), construct validity (correlations, analyses of variance), and responsiveness (effect size estimates) were evaluated. RESULTS: Cronbach alphas (range=0.86-0.91) and test-retest reliability (0.91) were highly satisfactory. Correlations with other measures provided evidence of convergent validity. sIBM patients able to walk without assistive devices scored significantly better on the sIFA (means=36.0-47.05) than those who require power mobility or wheelchairs (means=54.9-71.5), demonstrating the discriminating ability of the sIFA. Effect size estimates of responsiveness suggested mild functional progression. DISCUSSION: Psychometric analyses of the sIFA demonstrate satisfactory reliability, validity, and responsiveness. This article is protected by copyright. All rights reserved.