A 5-year-old boy with macrocephaly and mental retardation was referred for radiologic evaluation. After cranial CT and MR imaging, the diagnosis of mural type vein of Galen aneurysmal malformation was established by angiography. Two weeks later, preembolization angiography revealed complete thrombosis of the malformation. Although it is a very rare event, vein of Galen aneurysmal malformation may spontaneously thrombose following diagnostic angiography. Possible effects of contrast media on thrombosis were discussed.