PURPOSE: To evaluate the CT features of 15 patients with primary colorectal signet-ring cell carcinomas. MATERIALS AND METHODS: We retrospectively reviewed the CT scans of 15 patients (mean age, 44 years) with pathologically proven colorectal signet-ring cell carcinoma. The CT findings were evaluated in terms of site and length of the tumor, bowel wall thickening patterns, perirectal or pericolic infiltration, the presence or absence of colonic obstruction, and metastasis to other organs. RESULTS: The tumors were located in the rectum in nine patients, the sigmoid colon in one, the hepatic flexure in one, the transverse colon in one, the ascending colon in two, and the cecum in one. Tumor length ranged from 3.0 to 10.0 (mean, 6.1) cm, with a mean thickness of 2.1 cm. CT revealed concentric bowel wall thickening in all patients, and this was 'even' in eight and 'uneven' in seven. A target appearance was noted in four, perirectal or pericolic infiltration was moderate to severe in 12, and colorectal obstruction was seen in six. With regard to patterns of tumor spread, lymphadenopathy was noted in 13, invasion of adjacent pelvic organs in five, peritoneal carcinomatosis in four, liver metastasis in two, and periureteric metastasis in one. CONCLUSION: When CT shows a long section of concentric bowel wall thickening and a target sign, especially when such findings occur in the rectum and in young patients, primary signet-ring cell carcinoma should be included in the differential diagnosis.