Background. Hepatocyte nuclear factor-4 (HNF-4) plays a central role in the differentiation process of hepatic cells. We investigated the effects of an overexpression of HNF-4 on hepatic progenitor cells isolated from a fetal mouse liver and transplantation of the cells in a mouse model of liver fibrosis. Methods. Hepatic progenitor cells were isolated from the embryonic day 14.0 fetal mouse livers and were purified by magnetic cell sorting to remove the hematopoietic cells. We transfected adenovirus-mediated HNF-4 into the cells, and analyzed the expressions of the liver-specific functions using reverse-transcription polymerase chain reaction and Northern blotting. HNF-4-overexpressing hepatic progenitor cells were then injected into recipient mice, which were treated with dimethylnitrosamine and 30% partial hepatectomy. Results. After 5 days of culture, the cells located in the center of the aggregates were stained positive for albumin, but the peripheral cells for cytokeratin 19. Adenovirus-mediated HNF-4 gene transfer resulted in increases in the expressions of HNF-4, apolipoprotein (Apo)A1, ApoC3, and pregnane X receptor messenger RNA. The mice treated with HNF-4-transfected progenitor cells survived significantly longer than the control mice (P=0.004). The plasma levels of albumin, total cholesterol, and glucose were higher in the mice treated with cells transfected by HNF-4 than in the control mice. Conclusions. These findings demonstrate that adenovirus-mediated HNF-4 transfection induces the differentiation from hepatic progenitor cells to hepatic parenchymal cells in vitro. These cells may be useful as a source for cell transplantation in liver diseases.