Fifty-four patients with stenoses and occlusions of the extracranial internal carotid artery (ICA) demonstrated by intravenous digital subtraction angiography (IVDSA) were studied by extracranial duplex and transcranial Doppler. Two hundred healthy volunteers served as control group. Intracranial collateralization was demonstrated in 32 patients with ICA stenoses greater than 80% and occlusions. Twelve patients with unilateral and 8 patients with bilateral ICA obstructions greater than 80% showed collateral supply via the anterior (ACA) and/or posterior cerebral artery (PCA). In 5 cases of unilateral ICA occlusion and in 7 patients with bilateral ICA stenoses greater than 80% and occlusions, the ophthalmic artery and ACA and/or PCA served as collateral vessels. Five patients with ICA occlusions showed leptomeningeal anastomoses in the region of the postocclusive middle cerebral artery (MCA). All 54 patients showed normal time-mean flow velocity in the post-obstructive MCA. In unilateral ICA stenoses greater than 80% and occlusions, the pulsatility index in the ipsilateral MCA was reduced in comparison with the contralateral side, indicating that this parameter is more sensitive to hemodynamic effects of ICA obstructions than time-mean velocity.