Hemorrhagic cystitis (HC) is a major complication of bone marrow transplantation (BMT). We describe the clinical course and urological management of BK polyomavirus-associated HC in children after bone marrow transplantation. From 8/88 to 11/95, a total of 117 consecutive pediatric patients received BMT. Nine patients (7. 7%) developed HC after transplantation. HC in all 9 patients was characterized by late onset (day +24 to +50 post-BMT), long duration (1-7 weeks) and the excretion of BK polyomavirus in the urine as confirmed by electron microscopy, DNA hybridization and PCR techniques. Six children developed mild HC (grade 1-2) and were treated successfully by hyperhydration. In 3 patients, severe HC (grade 3-4) over 6 weeks required surgical interventions. In these 3 patients, cystoscopy revealed circumscript papulous tumors as the source of hematuria. Severe and persistent hematuria required blood transfusions, insertion of large suprapubic catheters and permanent bladder irrigation because of recurrent blood clot retention. Attempts to stop the hematuria in 2 of these patients by coagulation and laser vaporization (Nd:YAG) failed to stop the bleeding. Differential diagnosis of hematuria after BMT includes urinary tract infection, cyclophosphamide-induced chemical cystitis and bleeding due to BMT-induced thrombocytopenia. With the increasing number of BMTs in children, urologists may be confronted with BK polyomavirus-associated HC and must consider this in the differential diagnosis of hematuria after BMT.