Transforming growth factor-alpha (TGF-alpha) is a cytokines related to cell proliferation and transformation. Immunoreactive TGF-alpha protein is expressed in regenerating hepatocytes and interlobular bile ducts as well as in hepatocellular carcinoma. Although TGF-alpha is thought to play an important role in the intrahepatic biliary tree, its role in cellular physiology is poorly understood. This study investigates the expression of TGF-alpha and its messenger RNA (mRNA) in various hepatobiliary diseases. The authors showed by immunohistochemistry that TGF-alpha and its receptor, epidermal growth factor receptor (EGFR), were expressed in interlobular bile ducts, proliferating bile ductules, and most hepatocytes in various hepatobiliary liver tissues. They also showed by Western blot analysis that TGF-alpha protein was present in hepatic bile samples obtained from patients with obstructive jaundice. In situ hybridization showed that TGF-alpha mRNA was localized in hepatocytes of some pathological liver tissues, but it was absent in biliary epithelial cells of the same tissues. These findings suggest that TGF-alpha protein is produced by hepatocytes, and hepatocyte stimulation occurred as autocrine growth regulation. The release of TGF-alpha into hepatic bile caused biliary proliferation and transformation through EGFR, present on the existing cell surface membrane of biliary epithelial cells.