Distinct mechanisms are believed to regulate growth of the liver during fetal development and after injury in adults, because the former relies on progenitors and the latter generally involves replication of mature hepatocytes. However, chronic liver injury in adults increases production of Hedgehog (Hh) ligands, developmental morphogens that control progenitor cell fate and orchestrate various aspects of tissue construction during embryogenesis. This raises the possibility that similar Hh-dependent mechanisms also might regulate adult liver regeneration. The current analysis of murine liver regeneration after 70% partial hepatectomy (PH), an established model of adult liver regeneration, demonstrated that PH induced production of Hh ligands and activated Hh signaling in liver cells. Treatment with a specific Hh signaling inhibitor interfered with several key components of normal liver regeneration, significantly inhibiting progenitor responses, matrix remodeling, proliferation of hepatocytes and ductular cells, and restoration of liver mass. These global inhibitory effects on liver regeneration dramatically reduced survival after PH. Conclusion: Mechanisms that mediate liver organogenesis, such as Hh pathway activation, are retained and promote reconstruction of adult livers after injury. (HEPATOLOGY 2010;51:1712-1723) H ealthy adult livers have enormous regenerative capacity. This permits recovery of normal tissue-specific functions and mass within weeks of 70% partial hepatectomy (PH) in humans. Liver regeneration proceeds more rapidly in rodents, which accomplish liver reconstruction within 7 to 10 days after PH. 1 Thus, rodents are often used as experimental models to investigate regenerative mechanisms. Such work has consistently demonstrated that striking increases in hepatocyte DNA synthesis occur within the initial 48 hours after PH, followed by smaller (but highly significant) increases in hepatocyte mitoses and eventual recovery of liver mass, leading to consensus that liver regeneration after PH relies largely on increased replication of mature hepatocytes. 2-5 Nevertheless, changes in expression of progenitor markers, such as alpha-fetoprotein (AFP) and Fn14, have long been acknowledged to occur during regeneration. [6][7][8][9] Severe inhibition of liver regeneration after toxic liver injury was recently reported to occur in mice with