To evaluate the internal construct validity and dimensionality of a new patient-reported outcome measure for people with traumatic brachial plexus injury (BPI) based on the International Classification of Functioning, Disability and Health definition of activity.Cross-sectional study.Outpatient clinics.Adults (age range, 18-82y) with a traumatic BPI (N=106).There were 106 people with BPI who completed a 51-item 5-response questionnaire. Responses were analyzed in 4 phases (missing responses, item correlations, exploratory factor analysis, and Rasch analysis) to evaluate the properties of fit to the Rasch model, threshold response, local dependency, dimensionality, differential item functioning, and targeting.Not applicable, as this study addresses the development of an outcome measure.Six items were deleted for missing responses, and 10 were deleted for high interitem correlations >.81. The remaining 35 items, while demonstrating fit to the Rasch model, showed evidence of local dependency and multidimensionality. Items were divided into 3 subscales: dressing and grooming (8 items), arm and hand (17 items), and no hand (6 items). All 3 subscales demonstrated fit to the model with no local dependency, minimal disordered thresholds, no unidimensionality or differential item functioning for age, time postinjury, or self-selected dominance. Subscales were combined into 3 subtests and demonstrated fit to the model, no misfit, and unidimensionality, allowing calculation of a summary score.This preliminary analysis supports the internal construct validity of the Brachial Assessment Tool, a unidimensional targeted 4-response patient-reported outcome measure designed to solely assess activity after traumatic BPI regardless of level of injury, age at recruitment, premorbid limb dominance, and time postinjury. Further examination is required to determine test-retest reliability and responsiveness.