Using duplex sonography, we have routinely evaluated the vertebral arteries as part of the carotid artery examination in 453 consecutive patients over a 6-month period. Sixty-two of these 906 vessels could not be adequately evaluated, primarily because these vessels lay too deep within the vertebral structures, resulting in a technical failure rate of 6.8%. For the remaining 844 vessels, 74.4% were considered normal by Doppler flowmeter spectral analysis. Angiographic correlation was available for 224 vessels; of 155 judged normal by duplex sonography, 144 (93%) were shown to be normal or have only mild atherosclerotic disease. Eleven vessels were considered either occluded or to have severe disease by duplex sonography, and angiography showed this to be the case in 10 (91%) vessels. Unusually strong vertebral artery flow was associated with hemodynamically significant carotid or contralateral vertebral atherosclerotic disease or subclavian steal 82% of the time. Twelve cases of subclavian steal, only one of which was symptomatic, were identified by duplex sonography; four of these were confirmed by angiography. Angiographic correlation was available for 229 vessels in which duplex evaluation showed vertebral artery flow to be moderately damped. In 11 of these (38%), angiography showed greater than 50% stenosis. Angiography judged the remaining 18 vessels in this group normal; these vessels may represent a small subgroup of patients with normal anatomy or only mild atherosclerotic disease, but with hemodynamic dysfunction that can be identified with the duplex technique.