A standardized measure of neurological dysfunction specifically designed for TBI currently does not exist and the lack of assessment of this domain represents a substantial gap. To address this, the Neurological Outcome Scale for Traumatic Brain Injury (NOS-TBI) was developed for TBI outcomes research through the addition to and modification of items specifically relevant to patients with TBI, based on the National Institutes of Health Stroke Scale. In a sample of 50 participants (mean age = 33.3 years, SD = 12.9) <or=18 months (mean = 3.1, SD = 3.2) following moderate (n = 8) to severe (n = 42) TBI, internal consistency of the NOS-TBI was high (Cronbach's alpha = 0.942). Test-retest reliability also was high (rho = 0.97, p < 0.0001), and individual item kappas between independent raters were excellent, ranging from 0.83 to 1.0. Overall inter-rater agreement between independent raters (Kendall's coefficient of concordance) for the NOS-TBI total score was excellent (W = 0.995). Convergent validity was demonstrated through significant Spearman rank-order correlations between the NOS-TBI and the concurrently administered Disability Rating Scale (rho = 0.75, p < 0.0001), Rancho Los Amigos Scale (rho = -0.60, p < 0.0001), Supervision Rating Scale (rho = 0.59, p < 0.0001), and the FIM (rho = -0.68, p < 0.0001). These results suggest that the NOS-TBI is a reliable and valid measure of neurological functioning in patients with moderate to severe TBI.