Idiopathic dural arteriovenous malformation which occurs in the posterior fossa uses predominantly transverse and sigmoid sinuses. Cavernous sinus comes next and others are rather rare. However, we have recently experienced such a rare case which was operated on and cured completely. The malformation was fed through the anterior ethmoid artery and drained to the cortical vein. The case was a 36-year-old male and admitted in our clinic for having headache and nausea as the chief complaints. He was diagnosed subarachnoid hemorrhage due to the following findings: CT scans showed a high density zone localized in the sylvian vallecula. Cerebrospinal fluid obtained by a lumbar puncture was found to be bloody. No neurological abnormality other than neck stiffness and positive Kernig's sign was observed. Under study of right carotid arteriography, dural AVM was evident. The anterior ethmoidal artery which branched out from the ophthalmic artery fed the AVM. The cortical vein which ran on the surface of the frontal base was its drainer via a small nidus. There was no abnormality seen on a left carotid arteriogram. Surgery was proceeded with the right frontal craniectomy in extra- and intradural approach. At first, anterior ethmoidal artery was cut at the cribriform plate extradurally. After dura was incised, both the nidus and drainer were coagulated intradurally. The nidus was located at inner surface of the dura. The arachnoid hemorrhage was thought to be caused by rupture of this drainer.(ABSTRACT TRUNCATED AT 250 WORDS)