We report a case of intractable hemorrhagic cystitis due to cyclophosphamide therapy for Wegener's granulomatosis. Conservative treatment, including bladder irrigation with physiological saline and instillation of prostaglandin F2 alpha, failed to totally control hemorrhage. We then used hyperbaric oxygen at an absolute pressure of 2 atm, 5 days a week for 8 consecutive weeks. The bleeding ceased completely by the end of treatment and the patient remained free of hematuria thereafter. No side effect was noted during the course of therapy. In future, this form of therapy can offer a safe alternative in the treatment of cyclophosphamide-induced hemorrhagic cystitis.