Vein of Galen aneurysms may be defined as direct arteriovenous fistulas between choroidal and/or quadrigeminal arteries and an overlying single median venous sac. Anatomic analysis of 23 cases of vein of Galen aneurysm and correlation with known embryologic data indicate that the venous sac most probably represents persistence of the embryonic median prosencephalic vein of Markowski, not the vein of Galen, per se. The frequent concurrent venous abnormalities are easily understood as (a) retention of fetal anatomical features and (b) frequent occlusions of the dural sinuses of the posterior fossa, especially the sigmoid sinuses.