A prenatally diagnosed aneurysm of the vein of Galen was presented in the fetus of a patient referred to our hospital at 31 weeks of gestation. Ultrasonography demonstrated polyhydramnios, cardiomegaly, dilatation of the right atrium and superior vena cava, tricuspid valve regurgitation, hydrocephalus, and a large hypoechoic mass with blood flow in the suboccipital region. Skin edema was shown thereafter. A 3262-g male was delivered by cesarean at 35 weeks of gestation. Computed tomography imaging demonstrated a large mass in the suboccipital region, after which thrombocytopenia appeared and the neonate died at 18 days of age.