BACKGROUND: Cluster headache is characterized by strictly unilateral head pain associated with symptoms of cranial autonomic features. Transcranial Doppler studies showed in most studies a bilateral decreased blood flow velocity in the middle cerebral artery. OBJECTIVE: To investigate whether there is a bilateral or unilateral extracranial vasodilation during spontaneous cluster headache attacks. DESIGN AND METHODS: In 9 cluster headache patients, we investigated the luminal diameter of the superficial temporal artery with ultrasound on the headache and headache-free side during and outside cluster headache attacks. RESULTS: During cluster headache attacks, the diameter of the superficial temporal artery on the painful side was greater, 1.48 mm, than the diameter on the nonheadache site, 1.14 mm (P < .01). Outside attacks, median diameters on the 2 sides were quite comparable, 1.34 vs 1.31 mm (P = .67). CONCLUSIONS: What was observed is most likely a general pain-induced arterial vasoconstriction (confer the decrease in diameter on the pain-free side) with an unchanged superficial temporal artery on the pain side because of some vasodilator influence.