Intra-operative ultrasonography was performed in 10 patients with apparently single malignant tumours of the liver without invasion of the portal or hepatic veins. Five patients had hepatocellular carcinoma associated with cirrhosis and 5 had a secondary liver cancer. At laparotomy other tumoral lesions in 3 cases, portal vein thrombosis in 2 cases and invasion of the left hepatic vein in 1 case were detected by ultrasonography. In 2 patients deep intra-hepatic tumours less than 2 cm in diameter, which were neither visible nor palpable, were also detected by this method. As a result of the examination, subsegmental resection was carried out in 3 cases and hepatic lobectomy in 2 cases. Intra-operative ultrasonography therefore is a useful method to locate invisible tumours and vascular structures, thus preventing vascular injuries during the operation, facilitating limited tumoral resections and generally improving prognosis in patients operated upon for malignant hepatic tumours.