A cyst is a fluid-filled sac. You may get simple kidney cysts as you age; they are usually harmless. There are also some diseases which cause kidney cysts. One type is polycystic kidney disease (PKD). It runs in families. In PKD, many cysts grow in the kidneys. This can enlarge the kidneys and make them work poorly. About half of people with the most common type of PKD end up with kidney failure.