Five patients with a diagnosis of rectal carcinoma were examined by magnetic resonance and computerized tomography (CT) to determine if magnetic resonance imaging (MRI) added any information to that obtained by computerized tomography. In each case, the imaging studies agreed with the surgical description of the tumor. The studies agreed on the presence of perirectal fat invasion and tumor position. Both modalities were positive for involved lymph nodes in the patient with metastatic disease to pelvic nodes, but both studies underestimated the number of nodes involved. These five cases were examined using two magnetic resonance scanners. The older one operated at 0.3 tesla (T) while the newer scanner operated at 1 T. Spatial resolution was improved with the newer scanner. In addition, the difference in signal intensity between tumor tissue and normal tissue was greater on the 1 T scanner. Tissue differentiation was useful in one case in which CT suggested extension of tumor into the vagina. The extension was confirmed on the MRI scan.