This investigation was designed to provide preliminary support for cognitive reserve theory in closed head injury (CHI), and demonstrate the effectiveness of using the Oklahoma premorbid intelligence estimate (OPIE) in research and clinical activities. Out of a possible 124 consecutive referrals, 26 patients (N=26) who underwent neuropsychological assessment following brain injury met study inclusion/exclusion criteria. Participants were included if they had exited post-traumatic amnesia (PTA), demonstrated uncompromised upper extremity use, displayed adequate verbal communication, and were judged capable of completing a full neuropsychological evaluation. Participants were divided into a closed head injury—negative premorbid history (CHI−) or closed head injury—positive premorbid history (CHI+) group based upon premorbid variables (e.g., history of alcoholism). Groups did not differ in terms of demographic variables or premorbid IQ. Despite having less severe head injuries, the CHI+ group had a greater pre–post difference for PIQ, and a significantly larger VIQ/PIQ discrepancy than the CHI− group. In conclusion, these findings suggest that the CHI+ group had diminished cognitive reserve secondary to the aggregate effects of premorbid insult, which resulted in greater cognitive decline following an additional stressor (i.e., CHI) than what might otherwise be expected from the head injury alone.