A 6-year-old boy had a sudden onset of rupture of an aneurysm of the middle cerebral artery. Narrowing of an artery proximal to the aneurysm was shown on an angiogram taken two days after the rupture. At operation, clipping of the aneurysm prevented further bleeding. This artery thrombosed spontaneously in the postoperative period. The outcome was favourable, and the patient returned to school life. Spontaneous thrombosis of an artery after subarachnoid haemorrhage in children is rare, and thrombosis of aneurysm is very rare. To our knowledge, this is the third reported case of spontaneous thrombosis of a cerebral saccular aneurysm with subarachnoid haemorrhage.