Promoter CpG island hypermethylation of critical genes is thought to play an important role in human colorectal tumorigenesis. In this study, we show that low levels of CpG island methylation occur in the normal intestinal mucosa of Apc(Min/+) mice and are increased in Multiple Intestinal Metaplasia (Min) polyps. We examined the interaction between CpG island hypermethylation and tumorigenesis by genetically modulating expression levels of the predominant DNA methyltransferase, Dnmt1, in Apc(Min/+) mice. We show that a combination of Dnmt1 hypomorphic alleles results in the complete suppression of polyp formation and an accompanying reduction in the frequency of CpG island methylation in both the normal intestinal mucosa and intestinal adenomas. These results suggest that sufficient DNA methyltransferase expression is a prerequisite for polyp formation and that hypomorphic alleles of Dnmt1 are not merely genetic modifiers but the first identified true genetic suppressors of the Min phenotype.