We review our experience with 18 consecutive patients who received intravesical carboprost tromethamine, an F2-α prostaglandin, for severe hemorrhagic cystitis following cyclophosphamide chemotherapy. Of the patients 16 were given cyclophosphamide for conditioning before bone marrow transplantation and 2 received the drug as cytotoxic therapy alone (dose range 3.6 to 15.8gm.). All patients had severe gross hematuria that was refractory to forced diuresis and to continuous saline bladder irrigation. The intravesical prostaglandin therapy was initiated only after significant transfusion requirements (greater than 1 unit packed red blood cells per day) and/or numerous catheter manipulations for relief of clot retention. Eligible patients underwent complete clot evacuation followed by intravesical instillation of 0.4 to 1.0mg. % carboprost tromethamine for 2 hours 4 times per day, alternating with continuous saline bladder irrigation for 2 hours. Six patients attempted an alternate protocol of 0.8 to 1.0mg. % carboprost tromethamine given by continuous saline bladder irrigation.Complete resolution of gross hematuria occurred in 9 patients (50%). Eight patients had a partial response, with decreased transfusion requirements noted. However, complete resolution ultimately required an alternative therapy (for example formalin or urinary diversion). One patient (6%) failed to respond and required formalin therapy on day 4 of carboprost tromethamine therapy. Decreased red blood cell transfusion requirements were noted during and after therapy when compared to pretreatment values. No changes in renal or bladder function were noted during the mean followup of 17 weeks (range 1 to 64 weeks). There were 3 cases of recurrent hematuria. Side effects were limited to bladder spasm in 14 of the 18 patients (78%), with no systemic complications. The results suggest that carboprost tromethamine is a useful beside therapy for hemorrhagic cystitis due to cyclophosphamide, and treatment appears to have minimal toxicity.