Preoperative detectability rates of hepatocellular carcinoma smaller than 5 cm in 113 cirrhotic patients were 91 percent by ultrasonography, 93 percent by computed tomography, and 92 percent by selective angiography. The combination of two methods produced detectability rates of 97 to 99 percent. One hundred three patients underwent various types of hepatic resection with the aid of intraoperative ultrasonography. Forty-four tumors (43 percent) were embedded in the liver, and these tumors were not detected by conventional surgical exploration. The detectability rates were 38 percent for hepatocellular carcinomas smaller than 2 cm, 57 percent for 2 to 3.5 cm tumors, and 71 percent for 3.5 to 5 cm tumors. All undetectable hepatocellular carcinomas were identified by intraoperative echography. The overall detection rate by this method was 98 percent, which was substantially higher than the preoperative rate. Intraoperative ultrasonography is a useful and indispensable method for performing atypical minor hepatectomy for the treatment of small hepatocellular carcinomas associated with liver cirrhosis.