Renal cysts are sacs of fluid that can form on and within the kidney. The cysts can be single or multiple and are also characterized as simple or complex. Simple cysts are thin walled and the fluid is watery. The cause of a simple cyst is unknown, although it is theorized that nephron tubules may form diverticula that grow out to form a cyst. This may occur with injury, dysregulation of cell growth or possible ischemia followed by signals that trigger cell proliferation. Simple cysts are more likely to appear as a person ages and are rare before age 20. Aging may cause natural loss of nephrons which may increase filtrate load in the remaining nephrons. Simple cysts cause hardly any symptoms and are generally found only when scans are done on the lower flank area for other reasons. Complex cysts are usually thicker and contain a more dense substance that may include blood, necrotic tissue, calcification, connective tissue, or tumor tissue. Complex cysts are more common with cancer of the kidney; if suspected, more tests including biopsy is required.