Intraoperative ultrasonography (IOUS) of the liver is a very useful tool during surgery for gastrointestinal tumors or hepatic tumors. In a prospective study of 82 elective operations for 57 gastrointestinal tumors and 25 hepatic tumors or metastases, we compared IOUS with preoperative ultrasonography, CT-scanning and MRI. IOUS supplied additional information in 38% of the operations. In 22% of the cases, this had a major impact on the intraoperative strategy with an alteration of the procedure. IOUS has a higher detection rate of intrahepatic lesions, discriminates better between solid or cystic nature of the tumor and offers the possibility of performing a safe and radical hepatic resection by its ability to localize lesions in relation to vascular structures. IOUS is a safe and simple technique which will be more cost-effective than extensive preoperative evaluation of the liver. It provides a more complete clinical staging of patients for whom adjuvant therapy is considered after resection of a colorectal carcinoma.