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Background: Hyperhidrosis is estimated to affect ~ 4.8% of the US population, and most patients experience a negative psychological impact. Here, we describe development and psychometric evaluation of a patient-reported outcome (PRO) measure to assess severity of axillary hyperhidrosis in clinical trials that meets current U.S. regulatory standards to support product approvals. Methods: Three rounds of hybrid concept-elicitation/cognitive-debriefing qualitative interviews were conducted in adults with clinician-diagnosed primary axillary hyperhidrosis, followed by similar interviews in children/adolescents. The draft measure included diary items for presence, severity, impact and bothersomeness (basis of the Axillary Sweating Daily Diary [ASDD]), exploratory weekly impact items, and a single-item Patient Global Impression of Change (PGIC). Phase 2 (adults only) and phase 3 (adults and children ≥9 years) clinical trial data were utilized to evaluate measurement properties of the resulting draft measure: floor/ceiling effects, nonresponse bias, test-retest reliability, construct validity, and responsiveness were assessed. The primary concept of interest was axillary sweating severity (ASDD Item 2); however, additional supportive concepts were explored to allow for development of a comprehensive hyperhidrosis measure. Results: Twenty-nine patient interviews were conducted (N = 21 adult and N = 8 children/adolescents), resulting in the ASDD (4 items, patients ≥16y) and child-specific ASDD-C (2 items ≥9y to <16y), as well as 6 Weekly Impact items and the PGIC (patients ≥16y). No floor/ceiling effects or response biases were identified. Consistency between hypothesized and observed correlation patterns between ASDD/ASDD-C items and other efficacy measures supported construct validity. Intraclass correlation coefficients supported test-retest reliability (0.91–0.93; Item 2). Large effect sizes (− 2.2 to − 2.4) demonstrated that the ASDD/ASDD-C Item 2 could detect changes in hyperhidrosis severity, supporting the measure's responsiveness. Patients perceiving a moderate improvement in symptoms on the PGIC experienced an average 3.8-point improvement on ASDD axillary sweating severity (Item 2); thus, a 4-point responder threshold was defined as a clinically meaningful change. Conclusions: Qualitative and quantitative evidence support the reliability and validity of the ASDD/ASDD-C and its use in the clinical evaluation of axillary hyperhidrosis treatments. Further evaluation of this measure in future research studies is warranted to demonstrate consistent performance across different axillary hyperhidrosis populations and in different study contexts. [ABSTRACT FROM AUTHOR]