Background and purpose– The acetazolamide (AZA) test is a well-accepted method for measuring the vascular reactivity of the cerebral arteries. In order to investigate the nature of this reactivity after long-term daily AZA treatment, the cerebral blood velocity (CBV) was measured using transcranial Doppler in patients under continuous AZA treatment after a single AZA 1 g intravenous (IV) dose. Methods– Thirteen patients (eight women, five men) on long-term daily AZA (750 mg/day, mean treatment duration 68 ± 12+ months) were included in the study. The CBV of the middle cerebral artery (MCA) and the basilar artery (BA), including the values of peak velocity, mean velocity and pulsatility index (PI) were measured. The examination was performed twice – with the initial IV administration of AZA and 20 min later. The results were compared with those of 10 age matched volunteers. Results– A consistent significant increase of CBV in the right and left MCA (P < 0.001 for both arteries) was found in all study participants. A highly significant decrease of peak CBV in the BA (P < 0.001) was found in the post-AZA velocities of the patient’s group. In the control group, a consistent significant increase in all post-AZA tests was demonstrated (P < 0.001). Conclusions– A mild elevation of blood velocity in the MCAs concomitant with a highly significant decrease of velocity in the BA was present in all examined patients. These patterns of CBV changes indicate the presence of a ‘steal phenomenon’ from the posterior to the anterior circulation and stress the necessity for caution when evaluating the indications for performance of the AZA test in patients under continuous AZA therapy.