Gangrenous cystitis is a rare condition, there are no typical symptoms or clinical findings, which hampers diagnosis and may worsen the prognosis. The authors report the case of a diabetic male patient, hospitalized because of poor general condition, dehydration and diffuse abdominal pain. The diagnosis was made by typical pelvic CT findings. Medical treatment must be intensive and begun as soon as possible. As in the present case, the situation may have a favorable outcome.