Human hepatocellular carcinoma (HCC) is one of the most common and aggressive malignancies. In order to identify genes involved in HCC progression, we conducted a differential display analysis and found osteopontin (OPN) to be overexpressed in HCC. OPN is known to be a secreted adhesive glycoprotein, associated with tumorigenesis and metastasis in several cancers. Quantitative polymerase chain reaction analysis of 30 HCC cases revealed the average ratio of OPN to glyceraldehyde-3-phosphate dehydrogenase in tumors to be significantly higher than that in the surrounding non-cancerous liver (4.7 +/- 1.6 vs 0.18 +/- 0.04, P = 0.0072). Immunohistochemistry confirmed the OPN protein was expressed mainly on cancer cells, and was positive in 12 of 30 HCC, most of which showed transcript overexpression. Both OPN transcript and OPN protein were significantly overexpressed in HCC with capsular infiltration, compared with HCC without capsular infiltration. Moreover, OPN-positive cancer cells were often dispersed in the periphery of cancer nodules and were adjacent to stromal cells. Although OPN overexpression was not related to vascular invasion or intrahepatic metastasis, OPN was suggested to play a role in HCC, especially in cancer-stromal interactions.