Ultrasonography plays an important role in the early detection of hepatocellular carcinoma. Ultrasonography detected 53% of 287 patients with small (less than or equal to 5 cm) hepatocellular carcinomas. Among 486 patients with hepatocellular carcinomas, sensitivities of intra-operative ultrasonography in detecting 451 small primary hepatocellular carcinoma nodules, intrahepatic metastasis in 330 patients with small hepatocellular carcinoma and 63 tumor thrombi were 98, 48 and 67%, respectively. The sensitivity of intra-operative ultrasonography in detecting small primary tumors was 10% better than ultrasonography, computed tomography, and angiography. The sensitivity of intra-operative ultrasonography in intrahepatic metastasis and tumor thrombus was two to three times better than pre-operative examination. Intra-operative ultrasonography was useful in detecting nonpalpable tumors and in guiding the transection of the liver, biopsy, and cryosurgery. Moreover, intra-operative ultrasonography made possible new hepatectomy procedures: systematic subsegmentectomy and hepatectomies which preserve the inferior right hepatic vein. Systematic subsegmentectomy guided by intra-operative ultrasonography resulted in better survival rates than the limited resection in patients with small hepatocellular carcinoma two years after hepatectomy; by the sixth year, this difference was significant (p less than 0.05). Ultrasonography and intra-operative ultrasonography are indispensable in the early detection, accurate diagnosis, operative guidance and postoperative care of hepatocellular carcinoma.