A completely thrombosed giant fusiform aneurysm of the peripheral anterior cerebral artery occurred in a 16-year-old female, with a familial and personal history of vascular headache for 4 or 5 years. She was admitted in a drowsy state with severe headache. Computed tomography revealed subarachnoid hemorrhage and intracerebral hemorrhage as a mottled and ring-like high-density area in the left paramedian frontal lobe not enhanced postcontrast. Left carotid angiography demonstrated an avascular mass and a 5 cm defect in the A3 portion of the anterior cerebral artery. The rupture of the giant fusiform aneurysm was confirmed intraoperatively.