A prospective study to determine the value of cerebrospinal fluid analysis in the differential diagnosis of meningitis was performed in 710 consecutively observed patients, both children and adults, who underwent lumbar puncture due to suspected central nervous system infection. Diagnoses included acute or presumed bacterial meningitis (n = 79), acute or presumed viral meningoencephalitis (n = 218), acute unclassified meningitis (n = 6), other infections of the central nervous system (n = 37), non-infectious neurological diseases (n = 76) and control patients (n = 294). The sensitivity, specificity and predictive values were determined for cerebrospinal fluid white blood cell count, total protein, lactate, glucose and C-reactive protein levels as well as the blood/cerebrospinal fluid glucose ratio. Determination of cerebrospinal fluid levels of lactate (greater than or equal to 3.5 mmol/l) was found to be superior to the other tests. The C-reactive protein level gave no additional diagnostic information when the lactate level was determined. The white blood cell count, and total protein and glucose levels were often unreliable tools for differential diagnosis, largely due to low sensitivity at realistic discriminatory limits. The study confirms that no cerebrospinal fluid test is fully reliable in distinguishing bacterial meningitis from other forms of meningitis.