Superficial temporal arterial (STA) dilation has long been associated with migraine pain, but it is still not clear if vasodilation alone is sufficient to produce pain. Extracranial-intracranial bypass (EC-IC) patients are known to exhibit STA dilation post-operatively. A group of 56 E.C.-I.C. patients was questioned about headache, during scheduled post-operative examinations. Routine pre-operative and post operative arteriograms of the STA were compared to verify vasodilation. Doppler sonograms were performed on eight of these patients to demonstrate systolic flow velocity changes and to evaluate velocity pulse curve profiles. Despite significant and sometimes marked arterial dilation, as well as pulsatile changes consistent with migraine, none of the subjects complained of post-operative headache. Implications of the results with regard to migraine, hypertensive and toxic headaches are discussed.