SummaryThe objective of this study was to determine whether inconsistent and erratic within-day and across-day performance is a symptom of mild to moderate traumatic brain injury (TBI), and to determine whether impaired consistency of performance can coexist, in the same patient, with intact or “normal” performance on single administrations of neuropsychological and other cognitive tests. The design was a matched-pair study in which a computerized cognitive test battery was administered 30 times over 4 days to all subjects. Performance patterns between TBI and control subjects were compared. Subjects also received traditional neuropsychological testing. The setting was a rehabilitation hospital outpatient department. The subjects were 12 adult volunteers, six with documented TBI and six with no history of TBI, neurologic illness, or injury. Control subjects showed consistent improvement of performance over days 1 to 4, whereas subjects with TBI showed erratic and inconsistent performance across days. In addition to inconsistent performance, some subjects with TBI showed worsening performance across days. The main outcome measures were performance on the Automated Neuropsychological Assessment Metrics (ANAM) battery and performance on traditional neuropsychological tests. Some patients with TBI in the study who have normal initial performance on traditional clinical neuropsychological tests and newly developed computerized cognitive tests show abnormalities of sustained performance. Such abnormalities are most apparent when performance is observed over multiple days, and are characterized by erratic and inconsistent across-day performance. Inconsistent performance was observed even in those subjects with TBI whose initial performance was equal to or better than that of the control subjects. Deficits in dynamic performance may explain why some patients with TBI who have excellent neuropsychological test performance nonetheless complain of functional decrement from premorbid ability.