We prospectively compared CT angiography (CTA) of the common carotid artery bifurcation using two different techniques with conventional angiography in patients with suspected stenoses of the internal carotid arteries in 20 symptomatic patients. Ten patients (Group 1) received 60 cc of contrast (medium 2 cc/sec) and CTA was acquired using 5 mm slices, reconstructed at 3 mm slice thickness. Ten patients (Group 2) received 90 cc of contrast (medium 3 cc/sec) and CTA was acquired using 3 mm slices reconstructed at 1 mm slice thickness. All CTA studies were postprocessed using maximum intensity projection algorithm. Stenoses were graded prospectively from CT angiograms and compared with selective conventional catheter angiograms. In Group 1, CTA overestimated the degree of narrowing in 9 of 10 stenoses proven by conventional angiograms. We interpreted 2 nearly occluded internal carotid arteries, 2 with moderate and marked stenoses, and 2 with no narrowings, but fibromuscular dysplasia on conventional angiograms as occluded on CTA, and 3 vessels as showing marked stenoses, not confirmed by angiography. CTA clearly depicted 1 mild stenosis, 4 normal bifurcations, and 6 occluded internal carotid arteries. In Group 2, CTA overestimated two stenoses; a correct diagnosis was made in 7 normal bifurcations, 3 mild, 2 moderate and 2 severe stenoses, 2 near occlusions, and 2 occlusions. Ulcerations were missed by CTA regardless of the technique utilized.