In a 3-year period, 110 patients with central nervous system infections of various causes were examined serially by means of transcranial Doppler sonography. In viral-induced infections, no changes of flow velocity in basal cerebral arteries were seen, whereas in bacterial meningitis, a significant increase of blood flow velocity in the middle cerebral artery was recorded. Its extent was mainly associated with the type of the infectious agent, most frequently observed in pneumococcal meningitis (77%). The increase was up to 100% of the baseline values and was reversible in all cases. All patients were offered full-scale neurointensive care, and all subjects with bacterial meningitis were fully heparinized.