OBJECTIVE: Patients with acute bacterial meningitis are often treated with sympathomimetics to maintain an adequate mean arterial pressure (MAP). We studied the influence of such therapy on cerebral blood flow (CBF). DESIGN: Prospective physiologic trial. SETTING: The Department of Infectious Diseases, Copenhagen University Hospital, Denmark. PATIENTS: Sixteen adult patients with acute bacterial meningitis. INTERVENTION: Infusion of norepinephrine to increase MAP. MEASUREMENTS: During a rise in MAP induced by norepinephrine infusion, we measured relative changes in CBF by transcranial Doppler ultrasonography of the middle cerebral artery, recording mean flow velocity (Vmean), and by the arterial to jugular oxygen saturation difference. In 10 out of 16 patients, serial measurements were performed until recovery or death. Individual autoregulation curves were analyzed by a computer program. Autoregulation was classified as impaired if Vmean increased by >10% per 30 mm Hg increase in MAP and if no lower limit of autoregulation was identified by the computer program; otherwise, autoregulation was classified as preserved. MAIN RESULTS: Initially, Vmean increased from a median value of 46 cm/sec (range, 30-87 cm/sec) to 63 cm/sec (33-105 cm/sec) (p < .0001), and arterial to jugular oxygen saturation difference decreased from 0.28 (0.16-0.51) to 0.21 (0.08-0.39) (p < .001) when MAP was raised from 69 mm Hg (55-102 mm Hg) to 110 mm Hg (93-129 mm Hg). CBF autoregulation was restored in eight of ten patients undergoing serial examination after 7 (range, 2-10) days. Six of these patients had an uncomplicated course, one had a protracted recovery, and one died. Autoregulation was not restored in two patients; one died and one had a protracted recovery. CONCLUSION: In patients in the early phase of acute bacterial meningitis, CBF autoregulation is impaired. With recovery from meningitis, the cerebral vasculature regains the ability to maintain cerebral perfusion at a constant level despite variations in MAP.