Familial adenomatous polyposis (FAP) is caused by a dominant mutation in the adenomatous polyposis coli (APC) gene. Individuals with FAP progressively develop adenomas and carcinomas of the colon and rectum. We developed a mouse model for this disorder by genetically modifying the Apc gene. The resulting mice Apc1638 progressively develop neoplasms in the colon and remainder of the gastrointestinal tract. In this study when Apc1638 mice were fed a Western-style diet, they developed an increased incidence of the end point of carcinomas and number of invasive tumors. The findings therefore demonstrated dietary modulation of carcinoma incidence in mice with a targeted mutation providing a model for the study of gene-environment interactions in cancer.