Twenty-one patients with rectal carcinoma were examined preoperatively in order to assess the accuracy of CT in recognizing the extraluminal extension of the tumor and its effect on treatment planning. Evaluation was done by barium enema, endoscopy, and standard CT augmented by nonreconstructed coronal and sagittal sections. The information was compared with the operative and pathologic findings. Correct staging was achieved in 19 out of 21 patients. In two patients, CT overestimated the extent of the disease. In two patients, small distant metastases detected at surgery were not appreciated; however, the extent of pelvic disease was accurately assessed. In 17 patients, CT added new important information unobtainable by standard techniques. This new information affected treatment planning in 10 patients.