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Kidney Cysts, round pouches of fluid, form on or in the kidneys. Most of the kidney cysts are simple kidney cysts and thin-walled sacs which are filled with fluid. They are very common in the human body and can be harmless. Some of the kidney cysts are so small that they are not seen without a microscope. But others can grow to be as large as a tennis ball. As they become larger, those cysts can press on nearby organs and cause pain.