Hydrocephalus is a buildup of fluid in the brain, or more precisely, cerebrospinal fluid in the ventricular system, which causes potentially damaging increased pressure in the head. Most children who have it are born with it. It occurs in one or two per 1,000 live births.

The ventricular system in the brain is made up of four chambers that hold most of the cerebrospinal fluid that is in the head. The two upper chambers are called the left and right lateral ventricles. They connect to the third chamber or ventricle, which drains into the fourth.

Cerebrospinal fluid is a water-like substance that acts to cushion the brain within the skull. It also filters waste from tissue in and around the brain.

The cerebrospinal fluid is made in the walls of the ventricles. The fluid flows from the upper ventricles down to the lower ventricles and then over the surface of the brain and around the spinal cord. The cerebrospinal fluid is absorbed over the surface of the brain into the bloodstream.