A kidney cyst is a fluid-filled sac that grows in your kidneys, which are the bean-shaped organs that filter wastes out of your bloodstream to produce urine. You might have a single cyst on one kidney or many cysts on both kidneys. There are two types of cysts: simple cysts and polycystic kidney disease. Simple cysts are individual cysts that form on the kidneys. They have thin walls and contain a water-like fluid. Simple cysts don’t damage the kidneys or affect their function. Polycystic kidney disease (PKD) is an inherited condition that causes many cysts to form on the kidneys. These cysts can damage the kidneys as they grow. Cysts are usually harmless.