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Summary Objective The Global Assessment of Severity of Epilepsy (GASE) Scale is a single-item, 7-point global rating scale designed for neurologist-report of overall severity of epilepsy in children. Building on previous preliminary evidence of its validity and reliability for research and clinical use, this study evaluated the GASE Scale's construct validity, reliability, and responsiveness to changes in severity of epilepsy. Methods Data used for the study arose from the Health-Related Quality of Life in Children with Epilepsy Study (HERQULES), a 2-year multicenter prospective cohort study (n = 374) with observations taken at baseline, and 6, 12, and 24 months after diagnosis. Construct validity and reliability were quantified using Spearman's correlation and intraclass correlation coefficient (ICC). Responsiveness was assessed using both distribution-based and anchor-based indices. Results The GASE Scale was at least moderately correlated (r ≥ 0.30) with several key clinical aspects and most strongly correlated with frequency and intensity of seizures and interference of epilepsy or drugs with daily activities (r > 0.30). Total variation in GASE Scale scores explained by seven core clinical aspects of epilepsy increased over time (R2 = 28% at baseline to R2 = 70% at 24 months). The GASE Scale had modest test-retest reliability (ICC range: 0.52-0.64) and was responsive to changes in clinical criteria (standardized response mean range: 0.49-0.68; probability of change range: 0.69-0.75; Guyatt's responsiveness statistic range: 0.56-0.84). The GASE Scale showed potential to discriminate "stable" and "changed" patients according to select criteria and to a composite score (area under the receiver operating characteristic [ROC] curve range: 0.50-0.67). Significance Results offer additional evidence in support of the GASE Scale's validity, reliability, as well as responsiveness to changes in severity of epilepsy in children. We conclude that the GASE Scale is a potentially useful tool for assessing the severity of epilepsy in both clinical and research settings.