Cerebral vasospasm is a major complication associated with subarachnoid hemorrhage. In spite of extensive research, the pathogenesis of vasospasm remains obscure, and clinical management has so far been extremely difficult. For the evaluation of the efficacy of any treatment and the timing of operation, the development and resolution of the arterial narrowing should be monitored. It is important to know the correlation between the change in flow velocity and the course of clinical symptoms. In this study, the blood flow velocity in the different arteries was measured at short intervals (at least every third day) with transcranial Doppler ultrasonography, in eighteen patients with ruptured cerebral aneurysm. Flow velocity changes began to accelerate from 3 days after surgery, and reached the maximum value between 7 and 10 days, with normalization occurring within the following 2 weeks. The changes showed a significant relationship to the severity of vasospasm, the side of the operative approach, and the administration of calcium antagonist. Based on our results, we categorized the flow velocity changes of vasospasm for clinical practice as follows. (1) A slight acceleration of up to 120 cm/s in MCA and 90 cm/s in ACA was defined as within the normal range. In this state vasospasm was never observed angiographically. (2) Flow velocity changes between 120 and 140 cm/s were a subcritical state. These changes were seen in vasospastic patients who did not develop ischemic deficits. (3) Flow velocity changes of over 140 cm/s in MCA and 100 cm/s in ACA were critical vasospasm, since all of the patients who developed ischemic symptoms were in this group. Velocity changes higher than 170 cm/s in MCA and 130 cm/s in ACA seem to indicate a critical condition with a tendency to develop into brain infarction. The comparison between the time course of the flow velocity changes and clinical status showed that in symptomatic vasospasm the increase in velocity occurred before the manifestation of clinical symptoms. The transcranial Doppler measurements help to identify those patients who have a high risk of developing neurological deficits due to vasospasm, and help to select those who would benefit from preventive treatment in asymptomatic stage.