Increased levels of expression of hepatocyte growth factor (HGF) and its specific receptor c-met have been shown in the liver of several benign and malignant pathologies, both in experimental models and humans. We investigated by immunohistochemistry the presence of both HGF and c-met protoocogene product (c-met pp) in 20 hepatocellular carcinomas (HCCs), 5 focal nodular hyperplasias (FNHs), 4 cases of fulminant hepatitis (FH), and 1 case of regenerated liver. The c-met protooncogene product was expressed in all cases with marked overexpression in the HCCs and in ductular metaplasia. HGF was detected in the Ito cells of all cases and in neoplastic hepatocytes of 9 of 20 HCCs (45%). The proliferative index of each lesion was evaluated by means of the polyclonal antibody anti-cyclin A. When the level of expression of HGF and c-met protooncogene product with the percentage of cyclin A+ nuclei were compared, the closest relationship was between c-met protooncogene product and cyclin A+ nuclei were compared, the closest relationship was between c-met protooncogene product and cyclin A. In 11 of 20 HCCs (55%), there wa no correlation between HGF positivity and cyclin A. This seems to suggest that, independently of the levels of native liver HGF, c-met protooncogene product is the most active modulator of liver cell proliferation.