A case of acute hemorrhagic cystitis caused by adenovirus type 11, which occurred in an allograft recipient 6 months after a living-related renal transplantation, is described. The patient lacked a neutralizing antibody to adenovirus type 11 before transplantation. Adenovirus type 11 was isolated from his urine and he developed a neutralizing and complement-fixing antibody to this virus. Although adenovirus type 11 isolates had been obtained from 2 of 18 renal allograft recipients, only 1 patient suffered acute hemorrhagic cystitis. Adenovirus type 11 may play a role in acute hemorrhagic cystitis in renal allograft recipients during immunosuppressive therapy.