Purpose/Objective: To apply multivariate base rate analyses to the National Institutes of Health Toolbox Cognition Battery (NIHTB-CB) to facilitate the identification of cognitive impairment in individuals with traumatic brain injury (TBI). Research Method/Design: In a multisite cross-sectional design, 158 participants who sustained a complicated mild or moderate TBI (n = 74) or severe TBI (n = 84) at least 1 year earlier were administered the NIHTB-CB. The NIHTB-CB is comprised of 2 crystallized cognition tests (reflecting premorbid ability) and 5 fluid cognition tests, measuring processing speed, memory, and executive functioning. Base rates for obtaining 0 to 5 low fluid cognition scores were calculated across a range of cutoffs for defining a low test score (≤25th to 5th percentiles). Base rates of low scores in the TBI sample were compared to the NIHTB-CB normative sample using diagnostic accuracy statistics. Results: The proportion of the TBI sample obtaining low scores decreased as the cutoff for defining a low score decreased. Individuals with lower premorbid cognitive ability, as measured by NIHTB-CB Crystallized Composite score, tended to produce more low scores on the NIHTB-CB fluid cognition tests, even when using fully demographically adjusted scores. Certain patterns of low scores were associated with TBI (defined as likelihood ratio >2.0), whereas others were nonspecific, occurring almost as often in participants without TBI. Conclusions/Implications: Premorbid ability stratified base rate tables provided in this article can guide researchers and clinicians in the interpretation of NIHTB-CB performance in adults with TBI.