Cerebrospinal fluid (CSF) lactate values were measured in 26 children with meningitis (12 bacterial, 9 aseptic, 5 partially treated) and five children with meningococcaemia without meningitis. A reference range (0.5-3.2 mmol/l) was established from 100 control children. Amounts of lactate were significantly raised in bacterial meningitis (mean 6.5, range 4.5-10.2) compared with aseptic meningitis (mean 2.6, range 1.1-4.0) but this finding gave little practical help as the bacterial origin of the meningitis was clear from other CSF findings. High values (5.7) in a case of tuberculous meningitis (TBM) suggest that the test may be helpful when other CSF findings are inconclusive. Unless the CSF lactate is raised, the test is of minimal value in partially treated meningitis (mean 3.4, range 1.4-6.2). The previously unobserved finding of increased CSF lactate in meningococcaemia without meningitis (mean 3.9, range 3.1-5.0) supports the view that raised CSF lactate values in bacterial meningitis are not solely due to the presence of neutrophils. Literature relating to CSF lactate is reviewed.