BACKGROUND: Wild-type (WT) mice fed a conventional high-fat/high-sucrose diet (HFHSD) rarely develop metabolic dysfunction-associated steatohepatitis (MASH) with HCC. Because mouse bile acid (BA) is highly hydrophilic, we hypothesized that making it hydrophobic would lead to MASH with HCC. METHODS: Eleven-week-old WT and Cyp2a12/Cyp2c70 double knockout (DKO) mice were divided into two groups, including one which was fed a normal chow diet, and one which was fed an HFHSD. Samples were collected after 15, 30, 47, and 58 weeks for histological, biochemical, and immunological analyses. RESULTS: In the HFHSD group, body weight gain did not differ in WT versus DKO mice, although HFHSD-fed DKO mice exhibited markedly accelerated liver inflammation, fibrosis, and carcinogenesis. HFHSD upregulated lipogenesis and downregulated fatty acid oxidation in both WT and DKO mice, which increased liver lipid accumulation and lipotoxicity. However, the increase in reactive oxygen species production and carcinogenesis observed in DKO mice could not be explained by abnormal lipid metabolism alone. Regarding BA metabolism, DKO mice had a higher hydrophobicity index. They exhibited an age-associated increase in chenodeoxycholic acid (CDCA) levels because of CYP8B1 activity inhibition due to the farnesoid X receptor activation. HFHSD further downregulated CYP8B1, presumably by activating the Liver X receptor. Liver CDCA accumulation was associated with increased inflammation, reactive oxygen species production, and hepatocyte FGF15 induction. Moreover, in noncancerous liver tissues, HFHSD appeared to activate STAT3, an oncogenic transcription factor, which was enhanced by a CDCA-rich environment. CONCLUSIONS: Here, we developed a new model of MASH with HCC using mice with human-like BA composition and found that HFHSD and elevated hepatic CDCA synergistically increased the risk of MASH with HCC.