Venous malformations are abnormal tangles of thin-walled blood vessels, separated by normal brain tissue. A malformation can sometimes constitute a single greatly dilated, tortuous vein. While venous malformations of the brain are quite common (the estimated incidence in the general population is 0.5%), they are usually benign and asymptomatic.

Symptoms are unusual, and treatment of venous malformations of the brain is seldom found to be necessary. In the unusual case of hemorrhage associated with a venous malformation, it is usually secondary to a nearby Vein of Galen malformation.

Venous malformations can also occur in many other organs of the body, including the skin.