A cyst on the kidney is a fluid-filled sac that can form on or in the kidney. Most kidney cysts are benign and are called simple kidney cysts. They are usually small, round sacs that have a thin wall and are filled with a watery fluid. Simple kidney cysts can occur with disorders that may impair kidney function, but more often, they are harmless and do not cause any symptoms. However, in some cases, simple kidney cysts can grow large enough and cause a dull pain in a persons back, side, or upper abdomen. These cysts can also get infected, causing pain, fever, and tenderness. They can burst, also causing pain.

Imaging tests such as ultrasound, computerized tomography (CT) scan, and magnetic resonance imaging (MRI) are used to diagnose simple kidney cysts. Blood and urine tests can also be done to determine if simple kidney cysts are impacting kidney function. If they are not causing any problems or symptoms, then your doctor may just monitor them periodically. If simple kidney cysts cause problems, then surgery might be needed. Simple kidney cysts that are causing symptoms or blocking the normal flow of blood or urine through the kidney may need to be treated using a procedure called sclerotherapy, where the cyst is punctured using a long needle, guided by ultrasound. The cyst is then drained and filled with an alcohol-containing solution to harden it.