C-Myb is a sequence-specific DNA binding protein that regulates the expression of genes involved in cell proliferation and differentiation. The present study was designed to elucidate the role of c-Myb in the pathogenesis of chronic liver disease by an immunohistochemical approach. In normal (control) livers, few or no hepatic cells were positive for c-Myb. In livers from patients with chronic viral hepatitis, positive staining for c-Myb was found not only in spindle-shaped mesenchymal cells in expanded portal areas, but also in perisinusoidal cells (PSCs) within liver lobules. In cirrhotic livers, a few PSCs within lobules were positive for c-Myb, while no staining was seen in fibrous septa. Immunoelectron microscopy revealed that c-Myb-positive PSCs displayed morphological features of hepatic stellate cells. Other sinusoidal lining cells including Kupffer cells and sinusoidal endothelial cells, as well as hepatocytes, were all negative for c-Myb. Dual c-Myb/α-smooth muscle actin (αSMA) staining revealed that more than 97% of c-Myb-positive cells were αSMA-positive. Moreover, dual c-Myb/proliferating cell nuclear antigen (PCNA) staining showed that approximately 70% of c-Myb-positive cells also expressed PCNA. The labeling index (LI) (number of c-Myb-positive cells/0.1 mm2) significantly correlated with serum transaminase concentrations and increased in parallel with the disease activity. However, the LI showed no correlation with the degree of fibrosis. These results suggest that c-Myb may be involved in stellate cell activation and proliferation in chronically diseased human livers, and that the level of c-Myb expression is associated with the activity of chronic hepatitis.