The c-myc oncogene plays an important role in tumorigenesis and is frequently deregulated in many human cancers, including gastrointestinal cancers. In humans, mutations of the adenomatous polyposis coli (Apc) tumor suppressor gene occur in most colorectal cancers. Mutation of Apc leads to stabilization of beta-catenin and increases in beta-catenin target gene expression (c-myc and cyclin D1), whose precise functional significance has not been examined using genetic approaches. Apc(Min/+) mice are a model of familial adenomatous polyposis and are heterozygous for an Apc truncation mutation. We have developed a model for examining the role of c-Myc in Apc-mediated tumorigenesis. We crossed c-myc(+/-) mice to Apc(Min/+) to generate Apc(Min/+) c-myc(+/-) animals. The compound Apc(Min/+) c-myc(+/-) mice were used to evaluate the effect of c-myc haploinsufficiency on the Apc(Min/+) phenotype. We observed a significant reduction in tumor numbers in the small intestine of Apc(Min/+) c-myc(+/-) mice compared with control Apc(Min/+) c-myc(+/+) mice. In addition, we observed one to three polyps per colon in Apc(Min/+) c-myc(+/+) mice, whereas only two lesions were observed in the colons of Apc(Min/+) mice that were haploinsufficient for c-myc. Moreover, reduction in c-myc levels resulted in a significant increase in the survival of these animals. Finally, we observed marked decreases in vascular endothelial growth factor, EphA2, and ephrin-B2 expression as well as marked decreases in angiogenesis in intestinal polyps in Apc(Min/+) c-myc(+/-) mice. This study shows that c-Myc is critical for Apc-dependent intestinal tumorigenesis in mice and provides a potential therapeutic target in the treatment of colorectal cancer.