The early diagnosis of postoperative bacterial meningitis (BM) may be difficult. CSF cultures may remain sterile. Clinical features and routine laboratory data often fail to give an evidence. As early antibiotic therapy is essential in such patients, a rapid diagnosis is required. Different authors proposed the D(-) isomer of lactic acid as an early and effective marker of infection in the body fluids (including CSF). D(-) lactate is produced by bacteriae and fungi; L(+) lactate may be produced also by human tissues in anaerobic situations. We conducted a prospective study in a neurosurgical intensive care unit to evaluate this technique for the diagnosis of meningitis following craniotomy. Fifty-four patients were included, 40 in group A (not infected or infected out of the CNS), 4 in group B (suspected BM), 10 in group C (BM with positive CSF cultures). No patient suffered from septicemia, haemodynamic or ventilatory instability, nor metabolic disorder. Clinical data, CSF and blood samples (cytology, conventional biochemistry, D(-) and L(+) lactate, bacteriology) were collected at inclusion and, in group B and C patients, at day 2, 5 and at clinical recovery. D(-) lactate measurements were performed with an enzymatic method adaptated from a Boehringer Mannheim kit (for determination in foodstuff). Statistics were based on the comparison of group A vs C patients. D(-) and L(+) lactate concentrations in the CSF were significantly higher in group C patients, and blood concentrations were similar.(ABSTRACT TRUNCATED AT 250 WORDS)