A prospective study was done on 34 patients using magnetic resonance imaging (MRI) and computed tomography (CT) preoperatively to stage patients with known rectal carcinoma. The study was done to determine the accuracy and clinical usefulness of CT and MRI. The Thoeni staging method was used. Twenty-four of 30 cases were staged correctly by CT. Sixteen of 27 were staged correctly by MRI. CT detected lymph node metastases in six of 15 cases with one false-positive. MRI detected lymph node metastases in two of 15 patients with one false-positive. CT was the preferred examination, and was useful in some cases. These cases included patients with small tumors who were considered for local excision and patients with extensive disease who were candidates for preoperative or intraoperative radiation treatment. MRI demonstrated extensive disease, as did CT in our later cases.