An aneurysm is an abnormal widening or ballooning of a section of a blood vessel. When an aneurysm occurs in the brain, it is called a cerebral aneurysm.

Aneurysms in the brain occur when there is a weakened area in the wall of a blood vessel. An aneurysm may be present from birth (congenital) or it may develop later in life. (For example, after a blood vessel is injured.)

There are many different types of aneurysms. A berry aneurysm can vary in size from a few millimeters to over a centimeter. Giant berry aneurysms can reach well over 2 centimeters. These are more common in adults. Multiple berry aneurysms are inherited more often than other types of aneurysms.

Other types of cerebral aneurysm involve widening of an entire blood vessel, or they may appear as a "ballooning out" of part of a blood vessel. Such aneurysms can occur in any blood vessel that supplies the brain. Trauma and infection, which can injure the blood vessel wall, can cause such aneurysms.

About 5% of the population has some type of aneurysm in the brain. Risk factors include a family history of cerebral aneurysms, and certain medical problems such as polycystic kidney disease and coarctation of the aorta.