OBJECTIVE To determine the accuracy of transcranial Doppler (TCD) ultrasound for evaluation of collateral supply through the circle of Willis in patients with internal carotid artery (ICA) occlusion. METHODS The evaluation of the collateral pathways through the circle of Willis with TCD ultrasound and with cerebral angiography was compared in 40 patients (30 men, 10 women; mean age, 55 +/- 9 years) in a total of 44 ICA occlusions of which 43 had a suitable ipsilateral temporal bone window for TCD examination. RESULTS By TCD, a patent anterior communicating artery is indicated by a reversal blood flow in the A1-segment of the anterior cerebral artery or by a prompt fall of blood velocity in the middle cerebral artery after compression of the nonoccluded contralateral carotid artery. In 42 of 43 instances of ICA occlusion, TCD and angiography agreed in the evaluation of a present or absent anterior communicating artery collateral supply. TCD's sensitivity was 95%, its specificity 100%. A collateral supply through the basilar artery was assumed with TCD when there was: (a) a basilar artery blood velocity of more than 70 cm/s; (b) a marked increase of basilar artery blood velocity after compression of the nonoccluded carotid artery; (c) an evident side-to-side asymmetry of the blood velocity of the posterior cerebral arteries with high blood velocity ipsilateral to the ICA occlusion. For evaluating the collateralization via the basilar artery, TCD and angiography agreed in 37 of 40 ICA occlusions. TCD's sensitivity was 87%, its specificity 95%. CONCLUSIONS TCD is a reliable tool for the evaluation of the collateral supply in patients with ICA occlusions.