Primary objective: To examine the relationship between serum concentrations of protein S-100β and neuropsychological functioning following severe traumatic brain injury. Design: Matched control group. Methods: Blood samples were taken within 12 hours of injury and then daily up to 7 days post-injury (n = 23). Within 2 weeks of emerging from post-traumatic amnesia (PTA), participants completed a battery of neuropsychological measures. These results were compared with a matched sample of healthy controls. Results: Early measurement of S-100 not only reflected overall brain injury severity, but also related to neuropsychological deficits, with higher serum concentrations associated with poorer performance across most cognitive domains. PTA duration, measured by the Westmead PTA Scale, was found to be the strongest predictor of S-100 concentration (R2, p<0.001). Conclusions: These findings show that measurement of serum protein S-100 may further aid in the identification of individuals with severe TBI who are likely to experience cognitive difficulties.