Hemorrhagic cystitis is defined as acute or chronic bleeding of the bladder. Patients with hemorrhagic cystitis often present with gross hematuria and dysuria. In severe forms, bleeding can lead to a significant decrease in blood hemoglobin levels. Hemorrhagic cystitis can occur in response to chemical toxins (cyclophosphamide, penicillins, busulfan, thiotepa, dyes, insecticides), viruses (adenovirus types 11 and 21 (Chapter 254), and influenza A), radiation, and amyloidosis. The polyoma BK virus, present latently in immunocompetent hosts, has been recognized as playing an important role in the development of drug-induced cystitis in immunosuppressed patients.