It was shown in children that serum procalcitonin was the best marker to use to differentiate bacterial from viral meningitis. To evaluate procalcitonin in the diagnosis of acute bacterial and viral meningitis, we conducted a prospective study including adult patients who were suspected of having meningitis and who were admitted to an emergency department. Cerebrospinal fluid (CSF) and serum levels of procalcitonin were measured in 105 consecutive patients. The diagnosis of meningitis was based on clinical findings, gram staining, culture, and chemical analysis of CSF. Twenty-three patients had bacterial meningitis, 57 had viral meningitis, and 25 did not have meningitis. Bacteriologic and chemical analysis of CSF did not allow correct differentiation of viral from bacterial meningitis. On the other hand, a serum procalcitonin level >0.2 ng/mL had a sensitivity and specificity of up to 100% in the diagnosis of bacterial meningitis. Serum procalcitonin levels seem to be the best marker in differentiating between bacterial and viral meningitis in adults.