BACKGROUND The value of serum and cerebrospinaL fluid (CSF) procalcitonin for differentiating between acute bacterial and viral meningitis was assessed and compared to other parameters which are usually used in clinical practice. METHODS 45 adult patients (20 with bacterial and 25 with tick-borne encephalitis, TBE) were included in this prospective study. RESULTS The median serum procalcitonin Level in patients with bacterial meningitis was 6.45 ng/ml (range 0.25-43.76 ng/ml) and in the group with viral meningitis 0.27 ng/ml (range 0.05-0.44 ng/ml). 11 patients with bacterial meningitis had an elevated procalcitonin concentration not only in serum, but also in CSF. A serum procalcitonin Level > 0.5 ng/ml had a positive predictive value for bacterial meningitis of 100% and a negative predictive value of 93%, while corresponding values for CSF procalcitonin were 100% and 74%, respectively. CONCLUSIONS Serum and CSF procalcitonin concentrations > 0.5 ng/ml appear to be a reliable indicator of bacterial central nervous system (CNS) infection, with maximal positive predictive values and high negative predictive values.