Background: The optimal arterial carbon dioxide tension (PaCO2) in patients with acute bacterial meningitis (ABM) is unknown and controversial. The objective of this study was to measure global cerebral blood flow (CBF), cerebrovascular CO2 reactivity (CO2R), and cerebral metabolic rates (CMR) of oxygen (O2), glucose (glu), and lactate (lac), in patients with ABM and compare the results to those obtained in healthy volunteers. Methods: We studied 19 patients (17 of whom were sedated) with ABM and eight healthy volunteers (controls). CBF was measured during baseline ventilation and hyperventilation with single-photon emission computed tomography (SPECT) (14 patients) and/or the Kety-Schmidt technique (KS) (11 patients and all controls). In KS studies, CMR was measured by multiplying the arterial to jugular venous concentration difference (a-v D) by CBF. Results: CBF did not differ significantly among groups, although a larger variation was seen in patients than in controls. CO2R was not significantly different among groups. At baseline, patients had significantly lower a-v DO2, CMR(O2), CMR(glu), and CMR(lac) than controls. CMR(O2) did not change between hyperventilation compared to baseline ventilation, whereas CMR(glu) increased. Conclusion: In patients with acute bacterial meningitis, we found variable levels of CBF and cerebrovascular CO2 reactivity, a low a-v DO2, low cerebral metabolic rates of oxygen and glucose, and a cerebral lactate efflux. In these patients, a ventilation strategy guided by jugular bulb oximetry and/or repeated CBF measurements may be more optimal in terms of cerebral oxygenation than a strategy aiming at identical levels of PaCO2 for all patients.