Two hundred ninety-one courses of high-dose chemotherapy (HDC) with bone marrow transplantation (BMT) in children with malignancies were reviewed in order to assess the incidence, clinical course, outcome, and predisposing factors of hemorrhagic cystitis. Hemorrhagic cystitis occurred in 19 HDC courses (6.5%). Three patients had grade I hematuria linked to thrombopenia, nine had grade II hematuria despite platelet levels greater than 50 x 10(9)/L, and seven had grade III hematuria with clots and bladder obstruction. Severe complications occurred in grade III patients, but no deaths were directly linked to the cystitis. Fourteen patients recovered within two to 120 days of onset. The other patients died before the cystitis resolved, either of a relapse of the malignancy or of infection. Predisposing factors were age (increased incidence in older children), conditioning regimen containing cyclophosphamide, previous vesical irradiation, association with prolonged aplasia, and hepatic complications. The role of busulfan was also probable. No viral agent was found.