Bladder cancer is a logical target for metformin chemoprevention. Activation of the mTOR pathway has been detected in bladder cancer, while inhibition of this pathway blocks bladder tumorigenesis (2,3) in animal models and is associated with durable response in some patients with advanced bladder cancer (4). These observations provide the rationale for mTOR-directed therapy with metformin as a potential strategy for bladder cancer chemoprevention.