background: Intraoperative hepatic ultrasonography (IOUS) has been used to accurately identify and localize hepatic tumors as an adjunct to hepatic resection and for the detection of occult liver metastases during primary resection of other gastrointestinal carcinomas. The face validity of IOUS to identify more lesions than conventional diagnostic modalities and the content validity of IOUS to change the planned surgical management has been assessed in a blinded, prospective manner. methods: Sixty-two patients were studied at two institutions by one surgeon. IOUS was compared with computed tomography (CT) angioportography in 30 patients undergoing planned hepatic resection (19 metastatic, 11 primary) and with conventional hepatic ultrasonography (± venous enhanced CT scan) in 32 patients undergoing primary excision of gastrointestinal carcinomas. results: Twenty of the 30 hepatic resections (67%) were changed or guided by IOUS as determined by the operating surgeon at the completion of the laparotomy. IOUS detected 26 more metastases (44%) in 10 of 19 patients (1 to 5 per patient). Two patients had preoperatively suspected metastases refuted by IOUS-guided biopsy. Eight of the 11 patients (73%) undergoing resection of primary carcinoma of the liver had the planned procedure changed or guided by IOUS. This included four hepatocellular carcinomas with more extensive involvement at the confluence of the hepatic veins and the inferior vena cava, necessitating resection with the aid of total vascular isolation. In 32 patients undergoing primary resection of gastrointestinal carcinomas, 5 patients (16%) had the stage of disease altered by IOUS when compared with conventional ultrasound (± venous enhanced CT scan). conclusions: The validity of IOUS is good. IOUS guided the intraoperative surgical management of two thirds of the patients undergoing hepatic resection when compared with CT angioportography. Intraoperative hepatic ultrasonography using a reproducible systematic approach can change the clinical management of patients undergoing hepatic resection for malignancy.