Cerebral blood flow (CBF) following subarachnoid haemorrhage varies according to the time of sampling, with regard to the time of bleeding, to clinical conditions of patients and to any possible occurrence of vasospasm. CBF is proportionally reduced upon clinical conditions, involves the totality of patients and represents a diffuse bilateral phenomenon, not dependent on the location of the aneurysm. Following events, such as vasospasm, may deteriorate haemodynamic conditions, especially when unstable. Thus, CBF monitoring is necessary and useful in these patients. The evaluation of this parameter, according to the metabolic demands, appears rather fundamental. Besides, during the first two weeks, all the available therapeutic strategies have to employed in order to achieve the best optimization of the CBF so as to avoid the occurrence or at least minimising the extent of ischemic cerebral lesions.