Magnetic resonance (MR) angiography was applied to the study of blood flow dynamics and evaluation of extracranial-intracranial arterial bypass function in patients who had received bypass surgery. Three-dimensional gradient-echo acquisitions with selective presaturation of individual vessels were used to determine the direction of blood flow. Presaturation causes signal loss within the territory supplied by the presaturated artery, without affecting vessels not coursing through the presaturation slab. The patency of arterial bypass graft could be demonstrated as properly with MR angiography as with conventional angiography. In addition to that, MR angiography with presaturation pulse clearly demonstrated the direction of blood flow and the territory supplied by arterial bypass graft. The results were correlated with those from transcranial Doppler sonography and conventional angiography. We concluded that MR angiography is a noninvasive useful method for the evaluation of dependency on post-surgical collaterals and bypass function.