In 20 comatose patients (Glasgow coma scale less than or equal to 6 at admission) with severe head injury, the cerebral metabolic rate of oxygen (CMRO2) was calculated as the product of the hemispheric cerebral blood flow (CBF) and the arterio-venous oxygen content difference (AVDO2). The hemispheric CBF was calculated by the intracarotid 133xenon washout method by stochastic analysis as the average of 16 regions, and the measurements were performed within 3 weeks after the acute trauma. Generally no significant correlation (P less than 0.05) between CMRO2 and CBF was found, either in the total number of paired observations, in studies of hyperaemia defined as CBF greater than or equal to 30 ml 100 g-1 min-1; or in studies with reduced flow (CBF less than 30 ml 100 g-1 min-1). However, in about 50% of patients subjected to repeated studies within days, CBF was positively correlated to CMRO2, and this correlation was observed independently of the CBF value. Hyperaemia was associated with a significant decrease in AVDO2, a significant increase in both absolute and relative CO2 reactivity, and a significant increase in ventricular fluid pH; but not to an increase in intraventricular pressure, mean arterial blood pressure or significant changes in ventricular fluid lactate or lactate/pyruvate ratio.