Cerebrospinal fluid (CSF) lactate was studied in 63 consecutive infants and children with suspected bacterial meningitis. Levels of CSF lactate above 3 mmol/l were present in 14 out of 16 children with culture-proven meningitis, 1 out of 9 infants with viral meningitis, and 2 out of 6 with partially treated meningitis. Thirty-three infants who had their CSF examined to exclude meningitis served as control. None had levels above 2.3 mmol/l. The sensitivity, specificity, and efficiency of the test was 87.5, 97.6 and 94.8, respectively. We conclude that the test is useful, simple and reliable.