Progenitor cells, termed oval cells, are involved in the pathogenesis of hepatocellular carcinoma (HCC) in animal models. By immunolabeling for c-kit and CD34 in human hepatitis B virus-associated cirrhosis with HCC (50 cases) and those with cirrhosis alone (10 cases), we found c-kit+ tumor cells in tumor tissue in 40 of 50 HCCs. The proportion was less than 0.1% of total tumor cell volume in most HCCs. Immunostaining for c-kit also was detected in sinusoidal endothelial cells in 43 of 50 HCCs. The incidence of oval cell occurrence in the adjacent nonneoplastic tissue in cases of HCC was high (44/50). The occurrence of oval cells, c-kit+ tumor cells, and c-kit+ sinusoidal cells in cases of human hepatitis B virus-associated HCC suggests that oval cell proliferation might be associated with the development of human hepatitis B virus-associated HCC. Furthermore, the c-kit+ sinusoidal cells might have a role in angiogenesis and progression of human hepatitis B virus-associated HCC.