Transforming growth factor alpha (TGF-alpha) is expressed at high levels in human hepatocellular carcinoma (HCC); it may be part of a mechanism by which hepatitis B virus (HBV) causes HCC. Research has continued in the analysis of the role of TGF-alpha in the etiology of HCC. Previous work in this laboratory had shown high levels of TGF-alpha expression in most human HCCs, and an up-regulation of the TGF-alpha gene in a hepatoblastoma cell line after transfection with the hepatitis B virus (HBV). Subsequently, studies were conducted to examine the expression of TGF- alpha by immunohistochemistry in benign liver tumors in 17 patients with focal nodular hyperplasia (FNH), as well as 16 patients with HCC. All of these patients were from Hungary. Overexpression was detected in 13 patients with HCC (81%) and in 12 patients with FNH (71%). In patients with FNH, TGF-alpha stained strongly in the proliferating ductular cells. In selected patients with HCC, staining of consecutively cut sections revealed the expression of TGF-alpha and hepatitis B surface antigen in the same hepatocytes of adjoining nontumorous liver, as previously reported by this laboratory in HCCs from the United States and China. Additional studies have been conducted in HCC patients from Japan to evaluate TGF-alpha expression in tumor areas of different grades of histologic severity from the same patients, presumably in situations where a more severe grade evolved during the course of the disease. Preliminary results indicate that regions or nodules of the tumors with more differentiated histology (presumably, less advanced portions) have greater TGF-alpha expression.