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Background and Purpose: The modified Rankin Scale (mRS) is the most widely used primary outcome measure in acute stroke trials. However, substantial interobserver variability impairs outcome assessment as well as reduces power of clinical trials. Guided by the International Classification of Functioning, Disability and Health, we developed a comprehensive, hierarchical assessment tool (miFUNCTION) to address the shortcomings of the modified Rankin Scale and deliver a more thorough understanding of disability following stroke. Methods: The initial construct validity of miFUNCTION was established in a pilot study of patients at an outpatient stroke prevention clinic that had been diagnosed with stroke within 60 days. To further assess criterion validity, miFUNCTION was compared against the modified Rankin Scale and other outcome measures within the Endovascular Treatment for Small Core and Anterior Circulation Proximal Occlusion With Emphasis on Minimizing CT to Recanalization Times (ESCAPE) trial. Logistic regression analysis with miFUNCTION as an outcome was used to demonstrate the beneficial effect of endovascular treatment. Results: The pilot study showed moderate inter-observer agreement (k = 0.585, p < 0.005) but near perfect correlation between miFUNCTION and modified Rankin Scale (ρ = 0.821, p < 0.05). The correlation of miFUNCTION and modified Rankin Scale was near perfect again in the ESCAPE trial (ρ = 0.944). Effect size of the multivariable models using modified Rankin Scale (adjusted odds ratio: 3.45, 95% confidence interval: 2.05–5.78) and miFUNCTION (adjusted odds ratio: 3.32, 95% confidence interval: 1.99–5.55) as an outcome measure for the ESCAPE trial patients was similar. Conclusions: miFUNCTION is strongly associated with the degree of disability following stroke both in an outpatient setting and a clinical trial. Further work remains to assess sensitivity to change and to improve the inter-observer reliability of the scale. [ABSTRACT FROM AUTHOR]