S100B is a calcium-binding protein released into the blood from astroglial cells due to brain injury. Some authors have described a correlation between S100B serum concentration and severity of brain damage. There is not much information about the accuracy of urinary S100B for predicting outcome after severe traumatic brain injury (TBI). 55 patients with severe TBI were included in the study. Blood and urine samples were drawn to determine S100B levels on admission and on the subsequent 24, 48, 72 and 96h. S100B concentrations (serum and urine) were significantly higher in patients who were dead a month after the accident compared to survivors. ROC-analysis showed that S100B at 24h post-severe TBI is a useful tool for predicting mortality (serum: AUC 0.958, urine: AUC 0.778). The best cut-offs for S100B were 0.461μg/L and 0.025μg/L (serum and urine respectively), with a sensitivity of 90% for both measurements and a specificity of 88.4% (serum) and 62.8% (urine). We can state that the determination of S100B levels both in urine and serum acts as a sensitive and an effective biomarker for the early prediction of mortality after severe TBI.