The subclavian steal syndrome (SSS) is often associated with occlusive disease involving the subclavian or innominate arteries, but an asymptomatic subclavian steal, called the "subclavian steal phenomenon" (SSP), is not uncommon. Though intracranial collaterals had been postulated as one of the etiologies for the SSP's being asymptomatic, little has been accomplished in the investigation of extracranial channels. To study the hemodynamic role of cervical collateral channels, an angiographical study was done in three cases with SSP. The three cases were admitted to the hospital because of carotid ischemic symptoms, such as right hemiplegia or sensory aphasia. Each case had a blood pressure difference between the two arms, but in all of them the past history or the exercise test was negative for vertebrobasilar or arm ischemia. On angiography, occlusions of the unilateral proximal subclavian artery, the left in case 1 and the right in case 2, or a tight stenosis of the innominate artery was found in case 3. In each case, the vertebral artery flow in the affected side was inverted, siphoning off from the opposite vertebral artery into the affected subclavian artery. In addition to the vertebral siphoning; muscular branches of ipsilateral external carotid origin in cases 1 and 2, or the thyrocervical trunk via the inferior thyroid artery in case 3 was also found to function as a collateral channel to the vertebral artery on the affected side.(ABSTRACT TRUNCATED AT 250 WORDS)