OBJECTIVE: To investigate the importance of loss of consciousness (LOC) in predicting neuropsychological test performance in a large sample of patients with head injury. DESIGN: Retrospective comparison of neuropsychological test results for patients who suffered traumatic LOC, no LOC, or uncertain LOC. SETTING: Allegheny General Hospital, Pittsburgh, Pennsylvania. PATIENTS: The total number of patients included in this study was 383. MAIN OUTCOME MEASURES: Neuropsychological test measures, including the visual reproduction, digit span, and logical memory subtests of the Wechsler memory scale (revised), the Trail Making test, Wisconsin Card Sorting test, Hopkins Verbal Learning test, Controlled Oral Word Association, and the Galveston Orientation and Amnesia test (GOAT). RESULTS: No significant differences were found between the LOC, no LOC, or uncertain LOC groups for any of the neuropsychological measures used. Patients who had experienced traumatic LOC did not perform more poorly on neuropsychological testing than those with no LOC or uncertain LOC. All three groups demonstrated mildly decreased performance on formal tests of speed of information processing, attentional process, and memory. CONCLUSION: The results of this study cast doubt on the importance of LOC as a predictor of neuropsychological test performance during the acute phase of recovery from mild traumatic brain injury. Neuropsychological testing procedures have been shown to be sensitive in measuring cognitive sequelae of mild traumatic brain injury (concussion) in athletes. The failure of this study to find any relationship between LOC and neuropsychological functioning in a large sample of patients with mild head trauma calls into question the assignment of primary importance to LOC in grading severity of concussion. This study also does not provide support for the use of guidelines that rely heavily on LOC in making return-to-play decisions. Continued research is necessary to determine the relative importance of markers of concussion in athletes.