BACKGROUND: The Huntington's Disease (HD) Everyday Functioning (Hi-DEF) is a new patient-reported outcome (PRO) instrument designed to measure the impact of cognitive impairment on daily functioning in the early stages of HD. OBJECTIVE: To assess the measurement properties and finalize item content of the Hi-DEF. METHODS: A cross-sectional, observational psychometric validation study was conducted among individuals with early stages of HD at 9 US centers of excellence. Rasch Measurement Theory (RMT) analysis of the initial draft version of the Hi-DEF (47 items) and subscales (i.e., 'Home', 'At work', 'Driving', and 'Communication') was conducted to examine measurement properties including sample-to-scale targeting, suitability of response scale (ordering of response thresholds), scale cohesiveness (item fit), local independence, and person fit. RESULTS: 151 participants (mean age 47 years (SD 12), 59% female) were included. Seven items were removed based on dependency and item fit. The remaining 40-item version of the Hi-DEF demonstrated good measurement properties. Across the four subscales, targeting ranged from 49-70% (72% full scale), reliability ascertained by person separation index ranged from 0.53-0.87 (0.92 full scale), response scales were ordered for 25-100% of items (75% full scale), 0-12% items displayed misfit (2% full scale), and 0-1% (2% full scale) item pairs displayed dependency. CONCLUSIONS: Our study supports the psychometric integrity of the Hi-DEF as a reliable and valid new PRO instrument designed to assess the impact of cognitive impairment on daily functioning in the early stages of HD. Future work will evaluate the external validity and utility in clinical trial applications.