In a retrospective study of 78 cases of adult meningitis, the CSF lactate was measured on the first spinal tap (ST); 25 had a bacterial meningitis, 28 a viral meningitis; 22 other cases had been on antibiotics prior to admission; 3 cases had meningitis of rare aetiology. The median CSF lactate level among the 25 bacterial cases amounted to 13.6 mmol/l (range: 3.5-24.5) whereas it remained low in the 28 viral cases: 2.7 mmol/l (range: 1.4-4.2). These differences are highly significant. The comparison of the CSF lactate level with the other tests routinely performed showed that the CSF lactate level had the highest sensitivity, specificity and predictive values. The CSF lactate level on the first ST had no prognostic value, but a rapid decrease of the CSF lactate during the treatment is indicative of good prognosis. Among the pretreated cases, a high lactate level could be an indication that bacteria were the causal agents. In conclusion, the measurement of the CSF lactate, quickly performed and inexpensive, is worth performing when a meningitis is suspected, as it appears to be the best way of distinguishing bacterial from non-bacterial meningitis.