Bladder cancer begins in the lining of the bladder, the organ that stores urine before it leaves the body. One of the standard treatments for high-risk bladder cancer that has not spread into the bladder muscle is bacillus Calmette-Guérin, commonly called BCG. When cancer returns after BCG treatment, doctors usually recommend removing the bladder. The phase 3 clinical trial tested a new immunotherapy called cretostimogene grenadenorepvec in people whose high-risk bladder cancer returned despite BCG treatment. Even so, the results suggest that cretostimogene grenadenorepvec could become an important new option for patients with high-risk bladder cancer that returns after BCG therapy.