A kidney cyst is a swelling filled with watery fluid that forms on one or both of the kidneys.
Kidney cysts are round, have a thin, clear wall and range in size from microscopic to around 5 cm in diameter.
These cysts can be associated with serious conditions that lead to impaired kidney function, but usually they are what is referred to as simple kidney cysts, which do not tend to cause complications.
Although kidney cysts do not usually cause symptoms, some may cause a dull pain in the back or upper abdomen if they grow large enough.
Occasionally, they become infected, in which case a person may develop a fever, as well as suffering from pain and tenderness.
They can also cause severe pain in the back or side if they burst. A cyst that blocks the normal flow of urine may cause the kidney to swell, which is referred to as hydronephrosis.