Objective To investigate the psychometric properties of the Mayo-Portland Adaptability Inventory-4 (MPAI-4) obtained by self-report in a large sample of active duty military personnel with traumatic brain injury (TBI). Design Consecutive cohort who completed the MPAI-4 as a part of a larger battery of clinical outcome measures at the time of intake to an outpatient brain injury clinic. Setting Medical center. Participants Consecutively referred sample of active duty military personnel (N=404) who suffered predominantly mild (n=355), but also moderate (n=37) and severe (n=12), TBI. Interventions Not applicable. Main Outcome Measure MPAI-4 Results Initial factor analysis suggested 2 salient dimensions. In subsequent analysis, the ratio of the first and second eigenvalues (6.84:1) and parallel analysis indicated sufficient unidimensionality in 26 retained items. Iterative Rasch analysis resulted in the rescaling of the measure and the removal of 5 additional items for poor fit. The items of the final 21-item Mayo-Portland Adaptability Inventory-military were locally independent, demonstrated monotonically increasing responses, adequately fit the item response model, and permitted the identification of nearly 5 statistically distinct levels of disability in the study population. Slight mistargeting of the population resulted in the global outcome, as measured by the Mayo-Portland Adaptability Inventory-military, tending to be less reflective of very mild levels of disability. Conclusions These data collected in a relatively large sample of active duty service members with TBI provide insight into the ability of patients to self-report functional impairment and the distinct effects of military deployment on outcome, providing important guidance for the meaningful measurement of outcome in this population.