Pyourachus is an infection of urachal cyst which is one of congenital urachal anormalies. It should bedifferentiated from urachal tumor in adult, and abdominal desmoplastic round cell tumor, non-Hodgkin's lymphoma,and intra-abdominal desmoid tumor in children. The CT findings of pyourachus are characteristic to provide anaccurate preoperative diagnosis. The authors encountered a case of pyourachus in a 58-year-old male and report thecharacteristic CT findings. A large hypodense spherical mass with irregular thickened wall was present in themidline of the lower abdomen under the rectus abdominalis muscle extending from the dome of the urinary bladder tothe level of the umbilicus.