Forty-three patients with vein of Galen aneurysmal malformations (VGAM) referred to us for endovascular treatment between 1985 and 1990 and 335 additional cases published in the literature were reviewed with particular attention to the presence of ventricular enlargement and outcome after shunting. Hydrocephalus was the second most frequent symptom (46.8%); it is more frequent in infants (73%) than in children, adults (30%) or neonates (15%). Of the patients reported in the literature, 17.9% had undergone shunting. Within the shunted population there was an overall morbidity of 41% and a mortality of 10% (especially in the infant group). In our series 17 patients (39.5%) were shunted and a significant difference in the clinical outcome was noted between the shunted and the nonshunted group. Of the nonshunted patients, 66.6% were free of any neurological deficit or mental retardation and fewer than 5% presented with significant mental retardation. On the other hand, only 33.3% of the shunted patients had a favorable outcome and more than 15% developed significant mental retardation. Among the various causes of hydrocephalus in patients with VGAMs, such as obstruction of the aqueduct, subarachnoid hemorrhage, or ex vacuo hydrocephalus, high venous pressure may be of particular importance. In this article a physiopathological interpretation of the hydrodynamics in VGAMs is developed and a speculative explanation for CSF disorders related to ventricular shunting proposed. Treatment of hydrocephalus in VGAMs can be achieved through obliteration of the malformation or at least diminishing the venous pressure; surgical ventricular shunting does not have to be the first treatment of hydrodynamic disorders associated with VGAMs, especially in infants.