Transcranial Doppler sonography (TCD) flow velocities and cerebral blood flow (CBF) measurements were evaluated in 14 patients who had suffered a major aneurysmal subarachnoid haemorrahge (SAH). Cerebrovascular reactivity to hypocapnia, was evaluated simultaneously by the two methods. The measurements were performed under general anaesthesia preoperatively, within 72 hours after the bleed, during normocapnia and hypocapnia. There was poor correlation between absolute values of hemispheric CBF and corresponding TCD mean flow velocity. Controlled hyperventilation was associated with a significant decrease in CBF as well as TCD flow velocity (p<0.001). In terms of reactivity indices the correlation between the two methods was poor and not significant (r=0.33,p= 0.09). The principal differences between the methods are discussed as well as the application of TCD in the evaluation of cerebrovascular reactivity.