We report a case of a giant serpentine aneurysm (GSA) of the middle cerebral artery (MCA) that underwent complete spontaneous thrombosis after angiography. The patient was a 24-year-old man who had an acute headache. CT and MR examinations revealed a large partially thrombosed aneurysm of the MCA with residual flow in the artery. Angiography demonstrated a GSA arising from the left MCA. Thirteen days later, a second angiography showed complete thrombosis of the aneurysm.