A kidney tumor is a mass of abnormal cells that forms within the tissues of the kidney. Your kidneys are located on either side of your body. Kidney tumors can be cancerous or noncancerous. Small kidney tumors may not cause any symptoms initially.

Kidney cysts are estimated to occur in 10 percent of healthy adults. These cysts range in size and are typically filled with clear or straw-colored fluid. They can occur spontaneously or be caused by a disease, such as polycystic kidney disease. Rupture of a kidney cyst is unusual but does occur.