BACKGROUND: The basilar artery (BA) may act as an important intracranial collateral to supply hypoperfused anterior circulation. We examined the basilar reserve capacity by using transcranial Doppler sonography (TCD) in patients with occlusive vascular diseases in the anterior circulation (OAC) to determine whether vasomotor reactivity (VMR) of the BA can be affected by the presence of OAC. METHODS: Twenty-five patients were given an MRI including an MRA for minor ischemic symptoms, headache, or dizziness and were divided into two groups according to the presence of OAC. The OAC included occlusions of either the internal carotid artery (ICA) or M1 segment of the middle cerebral artery (MCA). The VMR to hypercapnea was calculated by means of the breath-holding index (BHI). We compared the VMR of the BA between those with and without OAC. RESULTS: Patients with OAC (n=8) showed significantly reduced basilar VMR, compared to that of patients without OAC (n=17) (0.57+/-0.49 versus 1.36+/-0.47, P=0.001). The baseline mean flow velocity of BA was also higher in patients with OAC (70.6+/-24.2cm/s) than those without (38.8+/-11.5 cm/s). However, the baseline pulsatility index did not show differences between patients with and without OAC. CONCLUSIONS: Patients with OAC showed decreased VMR of BA, which can be an easily applicable and useful TCD index for assessing the hemodynamic status in patients with OAC.