Thirty-nine patients were examined by computed tomography (CT) prior to resection of primary rectal [23] or rectosigmoid carcinoma [16]. There were 3 patients in Stage II [thickened bowel (greater than 0.5 cm) alone], 12 in Stage IIIa (thickened bowel wall and invasion of adjacent tissue, muscles, or organs, but no extension to the pelvic side walls), 18 in Stage IIIb (extension to the pelvic side walls), 6 in Stage IV (pelvic tumor and distant metastases), and none in Stage I (intraluminal mass without wall thickening). CT detected all 39 rectal or rectosigmoid tumors (sensitivity = 100%) and had an overall staging accuracy of 92%. These results show that CT is useful and accurate in detecting and staging primary rectal and rectosigmoid tumors.