Sumatriptan, a selective 5-hydroxy-triptamine (5-HT1) receptor agonist, has been used recently in the treatment of acute migraine. Some in vitro experiments suggested that sumatriptan has vasoactive properties in vascular beds distinct from cerebral circulation. In view of this we investigated the vascular effects of the standard 6 mg subcutaneous (s.c.) dose of sumatriptan, on the surface areas of the head using thermography, a simple and reliable method for detecting temperature changes. The head temperature of 127 patients (double-blind), 102 migraines (52 during headache attack and 50 headache-free) and 25 healthy control subjects were evaluated using thermography in basal condition and 30, 60, 90, and 120 min after s.c. sumatriptan injection of placebo. During the entire observation period systemic blood pressure (SBP), heart rate (HR) and continuous electrocardiogram (ECG) were detected automatically. A significant head temperature decrease was observed after s.c. sumatriptan administration, in both healthy controls and migraine subjects; placebo administration did not show any change of temperature. In migraine patients during headache attack, head temperature reduction corresponded to the relief of headache symptoms. This vasoconstrictor effect detected with thermography is not isolated to cranial circulation but it is also systemic. In fact, we observed a significant increase (p < 0.05) in both systolic and diastolic systemic blood pressure. No significant changes in heart rate and ECG abnormalities were otherwise detected. These findings suggest that sumatriptan is effective in the treatment of migraine attack, but it must be used with caution in migraines with concomitant hypertension.