It is not clear if ventilation with oxygen increases brain tissue oxygen pressure (PO2) during ischaemia. We have measured brain tissue PO2, carbon dioxide pressure (PCO2) and pH during baseline anaesthesia and oxygen ventilation in non-ischaemic control patients (n = 9), patients with cerebral occlusive disease (n = 11) and patients with arteriovenous malformations (AVM, n = 12). The same anaesthetic treatment was given to all groups and anaesthesia was constant during the study. Arterial pressure, brain temperature and arterial blood-gas tensions were similar between groups. Under baseline conditions, brain tissue PO2 was mean 4.2 (SD 1.4) kPa in the controls and was 70% lower in patients with ischaemia and AVM. Patients with occlusive disease also had elevated tissue PCO2 and acidosis. During oxygen ventilation, PO2 increased to 7.5 (2.9) kPa in controls and this was 50% greater than the increase in the ischaemia and AVM patients. The results showed that baseline tissue oxygenation and increases in PO2 during hyperoxia were attenuated in patients with ischaemia or AVM.