The correlation of the preoperative staging by CT with the postoperative staging was prospectively investigated in 112 patients with carcinoma of the rectum. The influence of CT on the choice of surgical treatment was also proven. The evaluation of the infiltration of perirectal tissue and especially of other organs is possible with CT. According to TNM classification the pre- and postoperative staging showed identical results by conventional diagnostic methods in T1 in 7/16, in T2 in 22/38, in T3 in 37/49 and in T4 in 5/9 patients. With the additional CT identical results were found in T1 in 7/14, in T2 in 25/31, in T3 in 49/53 and in T4 in 13/14 cases. Thus, the preoperative staging turned out to be correct with CT in 94/112 cases (83.9%). By conventional diagnostic methods identical results were found in 71/112 patients (63.4%). The infiltration of other organs was suspected preoperatively in 24 cases with CT and was found intraoperatively in 30/112 (accuracy 94.6%, sensitivity 88%, specificity 96%). Metastases of lymph nodes were suspected in the tomograms in 32/49 patients (65.3%). By the differentiated interpretation of the tumor growth with special regard to the "Grenzlamellen" of the rectum the CT gives important information for planning therapy.