It is difficult to assess the natural history of intracranial vascular malformations because they are varied in nature, they are frequently silent clinically, they are often treated when they are discovered, and untreated lesions are not often followed in an organized way. Capillary telangiectasias are usually occult lesions of no clinical significance. Cavernous hemangiomas may cause seizures and may bleed, but the approximate yearly risks of bleeding and of death have not been determined. Venous angiomas seldom cause symptoms, with the exception that those in the cerebellum seem to have a propensity to bleed. Intracranial dural arteriovenous malformations (AVMs) may bleed and may cause brain injury if there is insufficient outflow into a dural venous sinus. The dural AVMs that drain into the cavernous sinus have a more benign course than those that drain into the transverse or sigmoid sinus. The aneurysm of the vein of Galen presents a different clinical picture and threat to health according to whether the patient is a neonate, an infant, or an older child. The AVM of the brain encountered in the adult usually presents with hemorrhage or seizures. An unruptured AVM has approximately a 2 to 3% risk of bleeding per year, with about a 1% risk of death per year. The mortality rate of the first hemorrhage is about 10%. Among the survivors, there is about a 6% chance of rebleeding during the 1st year and then approximately a 2 to 3% risk of bleeding per year subsequently. The mortality rate associated with a second hemorrhage is about 13%, and for subsequent hemorrhages the mortality is roughly 20%.