Acute cavernous sinus syndrome developed in a 44-year-old man after he had experienced symptoms resembling cluster headache for 3 weeks. The cause was determined to be the sudden appearance of a large saccular aneurysm of the intracavernous portion of the left carotid artery secondary to an acute inflammation of the left cavernous sinus. The absence of this aneurysm before the onset of the ophthalmoplegia was confirmed by a computed tomographic scan and angiogram performed only 1 week earlier. The patient was treated with antibiotics in combination with a short regimen of steroids, which resulted in a slowly developing thrombosis of the aneurysm, although the cavernous sinus syndrome remained. This case suggests several important aspects regarding the pathogenesis of mycotic intracavernous carotid aneurysms, the mechanism of an acute manifestation of the cavernous sinus syndrome by an intracavernous carotid aneurysm, and the relation of the intracavernous portion of the carotid artery to cluster headache.