OBJECTIVEThe present study was conducted to validate S-100 protein as a marker of brain damage after minor head injury.METHODSWe studied 50 patients with minor head injuries and Glasgow Coma Scale scores of 13 to 15 in whom computed tomographic scans of the brain revealed no abnormalities. Serum levels of S-100 protein were measured at admittance and hourly thereafter until 12 hours after injury. Magnetic resonance imaging and baseline neuropsychological examinations were performed within 48 hours, and neuropsychological follow-up was conducted at 3 months postinjury.RESULTSFourteen patients (28%) had detectable serum levels of S-100 protein (mean peak value, 0.4 &mgr;g/L [standard deviation, ± 0.3]). The S-100 protein levels were highest immediately after the trauma, and they declined each hour thereafter. At 6 hours postinjury, the serum level was below the detection limit (0.2 &mgr;g/L) in five (36%) of the patients with initially detectable levels. Magnetic resonance imaging revealed brain contusions in five patients, four of whom demonstrated detectable levels of S-100 protein in serum. The proportion of patients with detectable serum levels was significantly higher when magnetic resonance imaging revealed a brain contusion. In patients with detectable serum levels, we observed a trend toward impaired neuropsychological functioning on measures of attention, memory, and information processing speed.CONCLUSIONDetermination of S-100 protein levels in serum provides a valid measure of the presence and severity of traumatic brain damage if performed within the first hours after minor head injury.