To evaluate hemodynamics of the vertebral artery (VA) in subclavian steal syndrome (SSS) and subclavian steal phenomenon (SSP), blood flow velocities of the bilateral VAs were measured by duplex ultrasonography in four patients with SSS and eight patients with SSP. The reversal of flow in the VA was noted in both systolic and diastolic phases in all of the SSS group, and was recorded only in a systolic phase in all but one of the SSP group. The antegrade mean flow velocities in the VA on the unaffected side in the SSS group were significantly higher than those in the SSP group. This indicates that collateral blood flow through the VA in the SSS group is still insufficient to compensate the blood requirement of the upper extremity on the affected side. In conclusion, hemodynamics of the VA in the SSS group could be distinguished from those in the SSP group by duplex ultrasonography.