Based on the recent reseach, Procalcitonin (PCT) has been found in serum and the concentration of PCT is higher in bacterial diseases. Objective: To investigate the threshold level of procalcitonin in serum and cerebrospinal fluid in patients with bacterial meningitis. Method: This is prospective, a cross-sectional study from September 2009 to June 2010 at Department of Tropical Diseases in Cho Ray hospital.