The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by rapid division of the mature cells. Proliferation of the parenchymal cells, i.e. hepatocytes and bile duct epithelial cells is regulated by numerous cytokine/growth factor mediated pathways and synchronised with extracellular matrix degradation and restoration of the vasculature. Hepatocytes, which have been infused into the liver vasculature, do engraft in small numbers and can participate in the regeneration process. In animal models with a selection advantage transplanted hepatocytes can grow to large numbers and may substitute the recipient liver mass completely. Since the first hepatocyte transplantations in animals, a number of reports have appeared showing the beneficial effects of hepatocyte transplantation in different animal models and also in human liver disease. In the clinic hepatocyte transplantation is being tested for the treatment of metabolic liver disease and to decrease mortality in acute liver failure. We have applied hepatocyte transplantation in four children with severe neonatal urea cycle defects (UCD). UCD is considered a promising target disease for liver cell transplantation (LCT), which may be a less invasive alternative or supplementation to orthotopic liver transplantation. Cryopreserved hepatocytes were isolated under good manufacturing practice conditions. The patients (age 1 day - 3 years) received multiple intraportal infusions of cryopreserved hepatocytes from that same donor, a 9-day old neonate. Portal vein access was achieved surgically in two children, whereas the umbilical vein was suitable for interventional catheter placement in two neonates. All children showed metabolic stabilization during observation periods of 4 to 13 months. One child with prenatally diagnosed ornithine transcarbamylase deficiency died after 4 months from a fatal metabolic decompensation. Although hepatocytes can be obtained from liver resection in live donors, or from cadaveric liver donors, one of the major impediments to more widespread use of hepatocytes to treat liver disease is a significant shortage of hepatocytes suitable for cell transplantation. For this reason, different sources of hepatocytes other than those primarily isolated from adult livers are being investigated not only to be used for clinical transplantation but also to investigate the potential of liver cell therapy. Since the capacity to produce liver cells for experimental and therapeutic use is limited, stem cells of various origins have been studied as a renewable source of liver cells. In vitro induction of hepatic phenotypes has been demonstrated in embryonic and adult stem cells and “proof of principal” transplantations have been performed. However, little is known yet about the relative repopulation and tissue forming capacity of these cells in a controlled setting compared to primary adult hepatocytes. We demonstrate that hepatocytes with an adult phenotype show the highest capacity to repopulate a mouse liver. We will discuss the implications for the development of new differentiation protocols and the requirements for stem cell based therapies in liver diseases.