Although inappropriate activation of the Wnt/β-catenin pathway has been implicated in the development of hepatocellular carcinoma (HCC), the role of this signaling in liver carcinogenesis remains unclear. To investigate this issue, we constructed a mutant mouse strain, Apc^lox/lox, in which exon 14 of the tumor-suppressor gene adenomatous polyposis coli (Apc) is flanked by loxP sequences. i.v. injection of adenovirus encoding Cre recombinase (AdCre) at high multiplicity [10⁹ plaque-forming units (pfu) per mouse] inactivated the Apc gene in the liver and resulted in marked hepatomegaly, hepatocyte hyperplasia, and rapid mortality. β-Catenin signaling activation was demonstrated by nuclear and cytoplasmic accumulation of β-catenin in the hepatocytes and by the induction of β-catenin target genes (glutamine synthetase, glutamate transporter 1, ornithine aminotransferase, and leukocyte cell-derived chemotaxin 2) in the liver. To test a long-term oncogenic effect, we inoculated mice with lower doses of AdCre (0.5 × 10⁹ pfu per mouse), compatible with both survival and persistence of β-catenin-activated cells. In these conditions, 67% of mice developed HCC. β-Catenin signaling was strongly activated in these Apc-inactivated HCCs. The HCCs were well, moderately, or poorly differentiated. Indeed, their histological and molecular features mimicked human HCC. Thus, deletion of Apc in the liver provides a valuable model of human HCC, and, in this model, activation of the Wnt/β-catenin pathway by invalidation of Apc is required for liver tumorigenesis.