In a clinic-based study we examine the prevalence of germline MYH mutations in multiple colorectal adenoma patients. Homozygous and compound heterozygous germline MYH mutations were identified in 30% of patients, and 67% carried mutations other than Y165C and G382D. Our results indicate that MYH mutation screening in patients with multiple colorectal adenomas should include the entire MYH gene.Mutations in the MYH gene, a member of the base excision repair pathway, have been associated with the development of colorectal adenomas and cancer as an autosomal recessive disease. In a population-based study of colorectal cancer cases and healthy controls we demonstrate an association between germline MYH mutations and an increased risk of colorectal cancer, and provide evidence for a moderate increased risk in heterozygous carriers that suggests a codominant mode of transmission. We conclude that MYH mutations may account for 1.7% of all colorectal cancer cases in Ontario.