A renal transplant patient with severe hemorrhagic cystitis of idiopathic etiology was treated initially with intravenous administration of conjugated estrogen (1 mg./kg.), followed on day 2 and thereafter with 5 mg. per day orally for 3 weeks. Hematuria decreased in intensity within 10 hours and disappeared within 48 hours. Hematuria did not recur by 6 months after completion of oral doses of conjugated estrogen. Complications or other side effects were not observed. In our experience conjugated estrogen controls hematuria in patients with idiopathic hemorrhagic cystitis and this form of treatment must be considered in this condition.