We evaluated the hemodynamic features of the posterior circulation in patients with severe carotid stenosis by assessing and comparing cerebral vasomotor reactivity (VMR) in the middle cerebral (MCA) and vertebral arteries (VA) by transcranial Doppler and the Diamox (1 g acetazolamide i.v.) test. Sixty symptomatic and 111 asymptomatic patients with unilateral severe (>70%) internal carotid artery stenosis were studied. The VMR was 19.2 +/- 18.9% for the MCA ipsilateral to the stenosis and 27.3 +/- 17.4% on the contralateral side (P < 0.0001) for all patients. It was 18.2 +/- 23.2% for the VA ipsilateral to the stenosis and 19.7 +/- 21% on the contralateral side (P = NS). The symptomatic patients' VMR of the MCA on the side of stenosis and the opposite side were 19.2 +/- 17.6 and 29 +/- 17.2%, respectively (P < 0.03). The VMR of the VA remained similar (15.1 +/- 21 and 21.6 +/- 6%, respectively, P = NS). The asymptomatic patients' VMR of the MCA on the side of the stenosis was also lower (19.2 +/- 19.7 vs. 26.5 +/- 17.5% on the opposite side, P < 0.001). In contrast, the VMR in the VA was similar (19.8 +/- 21.4 and 18.7 +/- 19.5%, respectively, P < 0.6, NS). Thus, the VMR of the posterior circulation remained similar regardless of carotid stenosis and a symptomatic/asymptomatic course of carotid occlusive disease, suggesting an independent cerebral vascular reserve capacity of the posterior circulation.