Kidneys can often have cysts. Here is one text that discusses these, and a couple of links to refer to:

Renal cysts are sacs of fluid that form in the kidneys. They are usually characterized as "simple" cysts, meaning they have a thin wall and contain water-like fluid. Renal cysts become fairly common as people age and usually do not cause symptoms or harm. Because they rarely cause symptoms, renal cysts are most often found during imaging tests performed for other reasons.