PURPOSE: To evaluate the findings of angiography and high-resolution CT of the skull base in the patients with congenital absence or hypoplasia of the unilateral internal carotid artery. MATERIALS AND METHODS: Eight patients with congenital unilateral absence (n=6) or hypoplasia (n=2) of the internal carotid artery were included in this study. None showed symptoms related to the absence of the artery. All underwent selective arteriography and six underwent high-resolution CT of the skull base. The angio-graphic findings of the carotid artery and collateral pathways to the absent side, as well as the high-resolution CT findings of the bony carotid canal at the skull base, were evaluated. RESULTS: In all cases, intracranial collaterals were of the adult type. The anterior cerebral arteries were supplied via the anterior communicating artery in all patients, and the middle cerebral arteries via the posterior communicating artery in five. In two, collateral flows were supplied by both the anterior and posterior communicating arteries, and in four, high-resolution CT of the skull base showed remnants or sclerosis of the carotid canal. One patient showed a hypoplastic bony carotid canal, and in one, this canal was absent. Intracranial aneurysms were found in four patients; in three, thses were located at the anterior communicating artery, and in the other, at the posterior cerebral artery. In four of six patients with no internal carotid artery, the ophthalmic arteries were opacified via the middle meningeal artery. CONCLUSION: In cases involving congenital absence or hypoplasia of the internal carotid artery, differentiation between agenesis and aplasia may be based on the pattern of collateral circulation. High-resolution CT find-ings may suggest that this change has a congenital origin.