N-chlorotaurine, a weak antimicrobial oxidant produced by stimulated human leukocytes, was used to treat cystitis caused by an omniresistant Pseudomonas aeruginosa. A 21-year-old male patient was treated by repeated daily lavages of the urinary bladder with an aqueous solution of 1% N-chlorotaurine for one month. N-chlorotaurine was well tolerated; no local or systemic side effects could be detected. Despite killing of > 10(6) cfu/ml of bacteria within ten minutes in vitro and in vivo, it was not possible to eradicate the Pseudomonas infection obviously caused by inflammation of the upper urinary tract and perpetuated by intravesical concrements. Nevertheless, in actually localized infection, treatment with N-chlorotaurine might be successful because of its sufficient bactericidal action.