SYNOPSISA subcutaneous injection of 6 mg sumatriptan rapidly and effectively stopped attacks of cluster headache. After a time lag of 4–14 minutes (mean 7 minutes) pain dramatically dropped to zero within seconds to single minutes (mean 56 seconds). This rapid effect may indicate that mere vasoconstriction is the mechanism behind the beneficial effect of sumatriptan in cluster pain. The findings support a scenario in cluster headache where an inflammation in the cavernous sinus affects the sympathetic fibers traversing the cavernous region. This leads to the miosis, ptosis and forehead anhidrosis but also to a dilated internal carotid arterial tree distal to the lesion. The dilatation, in combination with an obliterated drainage of the cavernous sinus by the inflammatory process, leads to progressive stasis in the sinus, to cause the painful attack. The attack terminates when the enhanced load on the sinus is reduced by constriction of vessels supplying the sinus, as is achieved by administration of sumatriptan. The present observation that other accompanying symptoms during attacks (nasal congestion, rhinorrhea, lacrimation and swelling of eyelids) disappeared in parallel with the pain points to the possibility that these symptoms may be directly related to venous stasis or activation of pain fibers, rather than resulting from a primary parasympathetic activatio