Sixty-three patients with mobile rectal cancer were examined preoperatively with endorectal ultrasonography (EUS). The depth of infiltration and the presence of mesorectal lymph node metastases could be assessed in 53 patients. Doppler ultrasonography was performed in 16 cases with suspected lymph node enlargement in order to discriminate between lymph nodes and blood vessels. Tumour growth in the bowel wall was correctly estimated in 43 (81%) patients. The degree of spread was overestimated in five patients and underestimated in five. The evaluation of the mesorectal lymph node status was also accurate in 43 (81%) patients. Nine patients had one or several regional lymph node metastases, but the EUS revealed only some of the metastatic lymph nodes in each case. In the other 34, no lymph node metastases were found. In two patients the EUS was falsely positive since no lymph nodes could be demonstrated in the operative specimens. In eight patients the examination was falsely negative. EUS is considered to be an accurate method for preoperative assessment of tumour infiltration in the bowel wall as the risk of understaging was under 10%. Preoperative irradiation and surgery may be chosen based on the EUS-determined tumour extension into the rectal wall.