Based on clinical data, indicating that chloroacetaldehyde (CAA) is an important metabolite of oxazaphosphorine cytostatics, an experimental study was carried out in order to elucidate the role of CAA in the development of hemorrhagic cystitis. The data demonstrate that CAA after i.v. administration does not contribute to bladder damage. When instilled directly into the bladder, CAA exerts urotoxic effects, it is, however, susceptible to detoxification with mesna.