Mutations affecting phosphorylation sites in the β-catenin gene have been implicated in the development of human and rodent hepatocellular carcinomas (HCCs). To further investigate the involvement of this gene in hepatocarcinogenesis, we used several transgenic mouse models of hepatic tumors induced by overexpression of c-myc in the liver either alone or in combination with transforming growth factor (TGF) α or TGF-β1. Activation of β-catenin, as judged by the presence of mutations and/or nuclear translocation of the protein, was most frequent in liver tumors from c-myc (4/17; 23.5%) and c-myc/TGF-β1 (6/18; 33.3%) transgenic mice. However, it was very rare in faster growing and histologically more aggressive HCCs developed in c-myc/TGF-α mice (1/20; 5%). Administration of diethylnitrosamine, phenobarbital, or 2-amino-3,8-diethylimidazo[4,5-f]quinoxaline did not significantly affect the occurrence of β-catenin mutations. Notably, nuclear accumulation of β-catenin was observed only in adenomas and highly differentiated carcinomas with eosinophilic phenotype. Furthermore, preneoplastic lesions with eosinophilic phenotype frequently displayed focal nuclear positivity, colocalized with areas of high proliferation. In contrast, basophilic and clear-cell foci, as well as pseudo-glandular and poorly differentiated HCCs, exhibited a normal or reduced membranous immunoreactivity for β-catenin. These studies suggest that nuclear translocation of β-catenin and activation of Wingless/Wnt signaling may represent an early event in liver carcinogenesis, providing a growth advantage in a subset of hepatic tumors with a more differentiated phenotype.