A case of fungal aneurysm associated with presumed Tolosa-Hunt syndrome is reported. A 57-year-old man was admitted to our hospital with complaints of left blepharoptosis, headache and weight loss. Neurological examination revealed left ophthalmoplegia without facial hypesthesia. Visual acuity was normal. Laboratory studies showed raised ESR, 4+ glycosuria, and a blood sugar of 351mg/dl. Computerized tomography (CT) scan and left carotid angiography were considered normal. Left orbital venography showed no filling of the left cavernous sinus. Diabetic ophthalmoplegia was suspected by a neurologist. The patient was treated with insulin therapy, but visual acuity worsened, and hypesthesia was noted in the first and second divisions of the left trigeminal nerve. Subsequent CT scan demonstrated a high density lesion, which was homogeneously enhanced, in the left cavernous portion and the superior orbital fissure. The patient was presumed of Tolosa-Hunt syndrome, and prednine therapy (30mg/day) was started. On the second day after the administration of prednine, hypesthesia of the first and second division of the left trigeminal nerve improved. After 9 days of prednine therapy, the patient suddenly complained of severe headache, and lapsed into a coma. Massive hemorrhage with subarachnoid hemorrhage was recognized on the CT scan, with a marked midline shift to the right. The hematoma was immediately removed. A ruptured cerebral aneurysm was found at the bottom of the hematoma. The aneurysm was located in the distal portion of the left middle cerebral artery. Aneurysm clipping with external decompression and bilateral ventricular drainage was performed.(ABSTRACT TRUNCATED AT 250 WORDS)