A large proportion of non-FAP non-HNPCC patients with multiple colorectal adenomas have been reported to carry germline mutations on the MYH gene. Although the number of adenomas appears to be dependent on the number of mutated MYH alleles present in a patient, little is known on the relation of this number with cancer risk. Four hundred fifty-three APC-negative patients with more than five colorectal adenomas were screened for mutations on the entire coding sequence of the MYH gene. Pathogenic mutations were initially found in 74 patients without extradigestive tumors (22.5%) and subsequently in 75 at-risk relatives. Polyposis was more severe in cases with biallelic mutations. However, mutation copy number was correlated neither with the age at diagnosis of adenomas or adenocarcinomas, nor with the presence of a family history of colorectal tumors. Heterozygous and homozygous MYH mutation carriers were both at high risk for synchronous cancers (24% in colorectum and 16% in the upper gastrointestinal tract), but did not demonstrate an increased risk for extradigestive tumors. MYH-associated polyposis is a frequent inherited colorectal cancer predisposition with a strong dominance component. From age 25-30, MYH mutation carriers should be proposed an early screening program, which includes endoscopies of the upper digestive tract and the colorectum every 2 years.