A persistent trigeminal artery (PTA) has been found in a number of cerebrovascular diseases. A 73-year-old asymptomatic woman was noted to have a left PTA and left subclavian steal by catheter angiography. Carotid duplex revealed a peak systolic flow velocity of 294 cm/s in the internal carotid artery (ICA) and an ICA to common carotid artery ratio > 4, suggestive of a high-grade stenosis. Only a low-grade stenosis was identified by catheter angiography. The elevated flow velocities in the left ICA were attributed to increased collateral blood flow across the stenosis to the left PTA, which compensated for the subclavian steal. Transcranial Doppler found an alternating flow pattern in the basilar artery (mean flow velocity [MFV] = 18 cm/s) and left vertebral artery (MFV = 43 cm/s). During brachial hyperemia, the MFV increased by 178% in the basilar artery and 102% in the left vertebral artery. The data suggest that a PTA may compensate for subclavian steal and may have a protective hemodynamic role in this setting.