Cluster headaches can be mimicked by a spontaneous carotid artery dissection. We report a 45-year-old man with a spontaneous carotid artery dissection whose unilateral headache responded to sumatriptan. An oral dose of 50 mg of sumatriptan relieved 90% of the pain after 2 hours. A second dose the next day achieved similar results within 4 hours. The diagnosis of dissection was made later by magnetic resonance angiogram and conventional angiography. This case illustrates that a positive response to a triptan can not be used to distinguish the first attack of cluster headache from a carotid artery dissection.