A 42-year-old woman suddenly developed headache and nausea on July 26, 1991, and the computed tomography (CT) scan showed a moderate-sized hematoma in the left occipital lobe. After one month's conservative treatment, she had recovered to a neurologically intact state. Cerebral angiography demonstrated a giant arteriovenous malformation fed by enlarged branches of the left posterior cerebral artery as well as small branches arising from the middle cerebral artery, anterior cerebral artery and the meningeal branches of the middle meningeal artery and the occipital artery. Preoperative embolization was planned on February 24, 1992. During an attempt at catheterization of the basilar artery and the left posterior cerebral artery with a balloon catheter and a Tracker-18 catheter, the patient complained of an intensification of her headache, nausea and vomiting. So the embolization procedure was stopped. The CT scan taken immediately at that time showed a severe subarachnoid hemorrhage (SAH). She became comatose about 40 minutes later. CT scan taken next day revealed also a complication of the pontine hemorrhage. Neurologically, she had gradually recovered and could communicate with some simple words 3 months after SAH. The total removal of the AVM was performed on May 26, 1992. Postoperative course was uneventful. She showed rapid and remarkable improvement in her neurological state suggesting that the blood flow in the surrounding brain area had been corrected. A blood deficit had no doubt been caused when blood had been stolen by the giant AVM.(ABSTRACT TRUNCATED AT 250 WORDS)