Background: The ability of cerebrospinal fluid (CSF) lactate to distinguish between acute bacterial meningitis (ABM) and aseptic meningitis/encephalitis (AME) is debated. We assessed the diagnostic value of CSF lactate to discriminate between ABM and AME. Methods: We included 176 patients from a prospective adult cohort with neuroinfections. In total, 51 ABM and 125 AME patients with clinically and/or microbiologically diagnosed acute meningitis were examined with CSF-lactate and traditional markers for infection. Receiver operating characteristic (ROC) curves were used to assess diagnostic performance. Results: In CSF, lactate, leukocytes, fraction of neutrophils, protein and glucose ratio, were significantly different between the ABM and AME groups. CSF lactate had the best diagnostic value, with an area under the curve (AUC) of 0.976 (95%CI 0.966–0.997) and using a cut-off of 3.5 mmol/L a sensitivity of 96% and specificity of 85%. Antibiotic treatment before lumbar puncture had no significant effect on the AUC of CSF lactate. Conclusions: Compared to traditional CSF-markers, CSF lactate is more accurate to distinguish between ABM and AME.