A 61-year-old Japanese man had been treated for schizophrenia and showed a high level of prostate-specific antigen (PSA, 11.68 ng/mL) at a clinic. He underwent a prostate biopsy at another hospital and was diagnosed with prostate cancer. In August 2019, treatment with goserelin (subcutaneous injection) and bicalutamide (oral administration) was initiated at our hospital’s urology department. From January to February in 2020, intensity-modulated radiation therapy (IMRT; total of 60 Gy/20 Fr) was administered. The patient’s PSA level went down. Diarrhea began during this IMRT period, and bleeding began after the completion of the IMRT. At 14 days after the end of the RT, the patient was admitted to another hospital. He was hospitalized with a diagnosis of radiation colitis. He continued fasting, and an intravenous drip was given. Three days post-admission, colonoscopy revealed a deep ulcer in the colon, which led to the suspicion of UC. The next day, he was transferred to our Teikyo IBD Center. On physical examination, the patient's abdomen was flat and soft without tenderness or distension. The laboratory data were as follows: RBCs 356 × 104/μL (low), hemoglobin 9.9 g/dL (low), WBCs 9100/μL, platelets: 32.5 × 104/μL, total protein 4.7 g/dL (low), albumin 1.5 g/dL (low), and CRP 14.41 mg/dL (high). He passed bloody diarrhea and the number of stools was > 10/day. Contrast CT showed diffuse edema and wall thickening throughout the colon. Colonoscopy showed a widespread map-like ulcer, pseudopolyposis, and very easy bleeding in the colon but edematous inflammation with no ulcer in the rectum. The patient’s Disease Activity Index [] was 11. We diagnosed with severe UC that worsened rapidly with uncontrollable massive bleeding, which was considered an indication for surgery. Emergency surgery was performed on the day of the patient's transfer to our Center. The surgery (total colectomy and creation of an ileostomy) was performed as follows. The abdomen was opened by a midline incision of the entire abdomen. Edema, redness, hyperemia, and thickening in the colon were observed. Intraoperative endoscopy revealed a deep ulcer in the sigmoid colon, but the rectum was slightly inflamed with no ulcer. A cut-off between the sigmoid colon and the rectum was selected. After transection of blood vessels, the ileocolic artery and vein were preserved and the ileum was cut-off at the terminal ileum. An ileostomy was created in the lower right abdomen. The specimens showed an ulcer spreading extensively throughout the colon. The pathological report was UC in the active phase. The patient’s postoperative course was good, and he was discharged 26 days after the operation.