In 27 comatose patients with acute head injury, 45 paired studies of regional cerebral blood flow (rCBF) were performed before and after hyperventilation. In total 676 regions were studied, and rCBF was calculated as initial slope index using the intracarotid washout technique of 133 Xe. The tests were applied from one day to three weeks after the acute trauma. In total hyperventilation from PaCO2 averaging 4.8 to 3.5 kPa increased the frequency of regions with oligaemia defined CBF less than 20 ml/100 g/min from 5 to 16%. Before hyperventilation oligaemia was observed in 11 of 45 studies (9 of 27 patients); after hyperventilation the frequency increased to 21 studies (15 patients). The frequency of severe oligaemia (CBF less than 15 ml) increased from 0.1 to 3% of all regions, or from 2 to 8 of all studies (from 2 to 9 patients). The increased frequency of oligaemia after hyperventilation was correlated to a poor outcome (dementia, vegetative survival or death), where it was observed in 21% of all regions, in 16 of 26 studies and 11 of 15 patients, whereas the frequency in patients with a good recovery was found to be 7% of all regions and observed in 5 of 19 studies (4 of 12 patients). The high frequency of oligaemia after hyperventilation was associated to a low hemispheric CBF before hyperventilation, but not to the level of PaCO2, the level of intracranial pressure, cerebral perfusion pressure or CSF-pH or lactate.(ABSTRACT TRUNCATED AT 250 WORDS)