Most kidney cysts (fluid filled mass) are classified as “simple” and are very common. They are generally benign, and found incidentally during an ultrasound, CT or other scan performed for some other reason. Most often these simple cysts can be left alone with no worries. Rarely, a simple kidney cyst can enlarge enough to cause pain, and these can be treated by needle drainage or more permanently with surgery (usually laparoscopy with small incisions.) Some kidney cysts are classified as “complicated” and might have more worrisome or malignant features that warrant more comprehensive exams and treatments including surgery. Very rare are multi-cystic and poly-cystic kidneys that can in worst cases cause kidney failure requiring dialysis or kidney transplant.