A cyst is an abnormal sac containing either fluid or semi-solid matter.

Cysts can be found anywhere on the body. The most common locations are in the skin, breasts, eyelids, testicles, ovaries and joints. They're less commonly found in the bones, kidneys and liver.

Some cysts are very small, being less than the size of a pea. When this is the case, they usually go unnoticed.

If they're larger they're often noticed because of their appearance or owing to the discomfort or pain they cause.

Causes

Most cysts appear suddenly for no apparent reason and usually later in life.

Sebaceous cysts of the skin occur when the duct from the sebaceous gland gets blocked. The secretion from the gland continues but can't escape, so the gland swells and the cyst forms. These cysts may be as small as a pinhead or as big as a tennis ball.

Some cysts appear soon after birth. These congenital cysts result from an abnormality during development. This is the case with some cysts in the kidneys.

Diagnosis and treatment

Those cysts that can't be felt or seen are usually diagnosed when an x-ray or scan is performed.

Most cysts are benign and don't need treatment to remove them, unless they're causing problems.

This article was last medically reviewed by Dr Trisha Macnair in December 2007