In a prospective study in 49 patients with rectal carcinoma the correlation of pre- and post-operative staging by CT and endorectal ultrasound (EU) was compared according to TNM-classification. With CT the pre/postoperative results correlated in T1 in 9/10, in T2 in 10/16, in T3 in 13/15 and in T4 in 6/8 patients. By EU identical results were found in all T1, in 10/12 T2, in 17/20 T3 and in 6/7 T4 stages. Overall, pre- and postoperative identical results were found by CT in 38 and by EU in 43/49 patients. Overestimation of the tumor stage was similar with both methods: 5 by CT and 4 by EU. Underestimation of the stage was more often by CT in 5 than by EU in 2 cases. (CT: accuracy 77.5%, sensitivity 88.3%, specificity 94.4%; ES: accuracy 87.7% sensitivity 91.5%, specificity 97.8%). Criteria for interpretation are discussed. In early tumor stages the depth of tumor invasion can be better evaluated by EU. In late stages both methods give important information for surgical strategy.