The results and consequences of intra-operative ultrasonography of the liver were studied in 50 patients who had laparotomy for colorectal cancer. Compared with preoperative imaging techniques like US, CT and MRI, intra-operative US had a higher sensitivity for intrahepatic lesions. Especially in case that adjuvant therapy could have been considered, intra-operative US gave relevant information in 10 patients (20%) by altering the stage of the primary tumour. Four of the 50 patients showed more liver metastases at intra-operative US than detected by preoperative imaging techniques. Resectional therapy of liver metastases could be prevented in these four patients. We advise intra-operative US as routine for all patients undergoing laparotomy for colorectal cancer especially if adjuvant chemotherapy is considered. When surgery is scheduled extensive preoperative liver examination can be avoided if intra-operative US is available.