Bladder cancer is a common type of cancer that begins in the cells of the bladder. The bladder is a hollow muscular organ in your lower abdomen that stores urine. Bladder cancer frequently starts in the cells (urothelial cells) that line within your bladder. Urothelial cells are found in your kidneys and the tubes (ureters) that interface the kidneys to the bladder. Urothelial cancer can occur in the kidneys and ureters but it's much more common in the bladder.