The early growth response-1 transcription factor (Egr-1) is induced as part of the immediate-early gene expression response during early liver regeneration. In the studies reported here the functional significance of EGR-1 expression during liver regeneration was examined by characterizing the hepatic regenerative response to partial hepatectomy in Egr-1 null mice. The results of these studies showed that liver regeneration in Egr-1 null mice is impaired. Although activation of interleukin-6-STAT3 signaling, regulation of expression of hepatic C/ebpα, C/ebpβ, cyclin D, and cyclin E and progression through the first wave of hepatocellular DNA synthesis occurred appropriately following partial hepatectomy in Egr-1 null mice, subsequent signaling events and cell cycle progression after the first round of DNA synthesis were deranged. This derangement was characterized by increased activation of the p38 mitogen-activated protein kinase and inhibition of hepatocellular metaphase-to-anaphase mitotic progression. Together these observations suggest that EGR-1 is an important regulator of hepatocellular mitotic progression. In support of this, microarray-based gene expression analysis showed that induction of expression of the cell division cycle 20 gene (Cdc20), a key regulator of the mitotic anaphase-promoting complex, is significantly reduced in Egr-1 null mice. Taken together these data define a novel functional role for EGR-1 in regulating hepatocellular mitotic progression through the spindle assembly checkpoint during liver regeneration.