Cluster headache is a rare very severe disorder that is clinically well characterized with a relatively poorly understood pathophysiology. In this study patients with episodic cluster headache fulfilling the criteria of the International Headache Society were examined during an acute spontaneous attack of headache to determine the local cranial release of neuropeptides. Blood was sampled from the external jugular vein ipsilateral to the pain before and after treatment of the attack. Samples were assayed for calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), substance P and neuropeptide Y. Attacks were treated with either oxygen inhalation, sumatriptan or an opiate. Thirteen patients were studied of whom 10 were male and three female. All had well-established typical attacks of cluster headache when blood was sampled. During the attacks external jugular vein blood levels of CGRP and VIP were raised while there was no change in neuropeptide Y or substance P. Calcitonin gene-related peptide levels rose to 110± 7 pmol/l (normal: <40) while VIP levels rose to 20 ± 3 pmol/l (normal: < 7). Treatment with both oyxgen and subcutaneous sumatriptan reduced the CGRP level to normal, while opiate administration did not alter the peptide levels. These data demonstrate for the first time in vivo human evidence for activation of the trigeminovascular system and the cranial parasympathetic nervous system in an acute attack of cluster headache. Furthermore, it is shown that both oxygen and sumatriptan abort the attacks and terminate activity in the trigeminovascular system.