We studied a total of 50 recipients who had received allogeneic bone marrow transplantation (BMT) and evaluated both the presence of hemorrhagic cystitis (HC) and the urinary excretion of adenovirus. Twelve recipients developed HC and eight of these 12 patients excreted adenovirus type 11 at the onset of cystitis. Urine for virus isolation was attempted 30, 60 and 100 days after BMT. Among 137 specimens examined, eight were positive for adenovirus type 11. Of these eight samples, six were collected during HC; while in the 129 samples which were negative for adenovirus, only three specimens was collected during HC. Female patients, seropositivity for the antibody to adenovirus prior to BMT and acute graft-versus-host disease (grade 2-4) showed a significant impact on the risk of adenovirus HC. It may be said that adenovirus type 11 is one of the causative agents of HC in BMT recipients.