Thirty-four patients with nonresectable adenocarcinoma of the rectum, defined as tumor fixation at digital examination, were examined with MR. All 34 patients had, according to MR imaging, perirectal tumor growth. In 23 (68%) of the patients, the tumor has reached an adjacent organ. Eight of these patients had disturbances of the MR characteristics in the adjacent organ which proved to be due to overgrowth, i.e., to tumor invasion into these structures. In the remaining 15 patients, without disturbed MR characteristics, 7 had tumor overgrowth at laparotomy. When there was a visible space between the tumor and adjacent organs, there was no sign of tumor overgrowth at laparotomy, except in one case. In 24 patients, examined both before and after combined irradiation and drug therapy, tumor regression was registered after treatment. MR imaging seems to be useful in the assessment of resectability and to evaluate preoperative anticancer treatment in patients with nonresectable rectal carcinoma.