OBJECTIVE: A greater reduction in cerebral blood flow has been shown in poor-grade patients with aneurysmal subarachnoid hemorrhage. The present study is designed to evaluate cerebral vasomotor reactivity(VMR) using a transcranial doppler(TCD) with acetazolamide(AZ) challenge in those patients and to investigate its predicting value for outcome. METHODS: Firstly, we compared the effect of AZ and most extensively using CO2 stimuli(breath-holding method) on middle cerebral artery flow velocity(FV) in the same control adults(n=12) to examine the reproducibility of AZ test. Secondly, the changes of FV of lesional and contralateral hemispheres after injection of AZ, which was measured one week after early surgery of poor-grade patients with aneurysmal subarachnoid hemorrhage, were compared between patients who showed good outcome(Group 1, n=22) and poor outcome(Group 2, n=22). RESULTS: There was a close correlation between both stimulation methods, indicating a strong similarity of the vasodilatory effects of CO2 and AZ. The FV at rest(lesional: 71.5+/-38.6cm/s, contralateral: 49.4+/-22.6cm/s) increased significantly after administration of AZ(lesional: 90.5+/-40.1cm/s, contralateral: 66.5+/-25.4cm/s, P<0.01, Student's t test) in Group 1. However, all had an insignificant increase of FV after AZ challenge compared with the baseline values in Group 2. The difference between the values at rest and after AZ had an increasing tendency but it was not significant, between Group 1 and 2(repeated measures ANOVA, P=0.06). CONCLUSION: These results demonstrate that the assessment of VMR by the change of FV using TCD together with AZ test in poor-grade patients with aneurysmal subarachnoid hemorrhage seems clinically helpful for predicting surgical outcome.