Regional cerebral blood flow (rCBF) and cerebral autonomic vasomotor response were investigated before and after 6 days of dexamethasone treatment (24 mg/day) in six patients with brain tumor, under neuroleptic analgesia and relaxation. In the untreated patients, mean rCBF in the affected hemisphere was markedly reduced, particularly in the peritumoral brain tissue. Autoregulation as well as cerebrovascular reactivity to Paco2 were locally or generally impaired. Following dexamethasone treatment, (a) average hemispheric rCBF was significantly (+38%) improved and (b) cerebral vasomotor response was markedly or completely restored. The findings suggest that dexamethasone tends to counteract the factors altering the cerebral vasomotor function and serves to remove edema fluid around brain lesions, with subsequent improvement in brain function and neurological symptoms.