Bladder cancer most often begins in the cells (urothelial cells) that line the inside of your bladder, a hollow muscular organ in your lower abdomen that stores urine. Most bladder cancers are diagnosed at an early stage, when the cancer is highly treatable. But even early-stage bladder cancers can come back after successful treatment. For this reason, people with bladder cancer typically need follow-up tests for years after treatment to look for bladder cancer that recurs.