Forty-eight patients with subarachnoid haemorrhage were studied with repeated rCBF and CMRO2 measurements. Cortical rCBF was measured using xenon-inhalation technique. CMRO2 was calculated as AVDO2 x CBF. When first studied the 29 conscious patients showed relative hyperaemia with CBF at 50 ml and reduced CMRO2 at 2.17 ml. In the following week CBF decreased to 41. CMRO2 remained reduced and constant. The 19 unconscious patients showed initially pronounced reduction in CMRO2 to 1.26, followed by gradual increase to 1.73 in 4-5 days. Simultaneously CBF increased from 18 ml to slightly above 30 ml. In the conscious patients the early reduction in CMRO2 and the concomitant luxury perfusion may be explained by global ischaemia because of very high ICP at the time of the haemorrhage. The reduced CBF in the unconscious group could be due to increased ICP, as ventricular drainage increased CBF to levels of relative hyperaemia as demonstrated in one case. As no decrease in CMRO2 was seen during the first 2 weeks, it is suggested that ischaemia at the time of aneurysm rupture is the most important single factor in reduction of global CMRO2.