A desmoid tumor was found in a patient with Gardner's syndrome. The tumor was visualized by CT before and after contrast injection, by radiographic examination of the small bowel, and by sonography. CT demonstrated a heterogeneous mass in the left lower abdomen with attenuation values ranging from 23 to 72 HU on native scans, enhancing to 123 up to 172 HU after injection of contrast medium. This examination also demonstrated the extent of the tumor and its exact dimensions. The radiographic examination of the small bowel completed the diagnosis and showed no involvement of the small bowel. This diagnosis was confirmed by laparotomy.