In 10-30% of patients with classical familial adenomatous polyposis (FAP) and up to 90% of those with attenuated (<100 colorectal adenomas; AFAP) polyposis, no pathogenic germline mutation in the adenomatous polyposis coli (APC) gene can be identified (APC mutation-negative). Recently, biallelic mutations in the base excision repair gene MYH have been shown to predispose to a multiple adenoma and carcinoma phenotype. This study aimed to (i) assess the MYH mutation carrier frequency among Swiss APC mutation-negative patients and (ii) identify phenotypic differences between MYH mutation carriers and APC/MYH mutation-negative polyposis patients. Seventy-nine unrelated APC mutation-negative Swiss patients with either classical (n=18) or attenuated (n=61) polyposis were screened for germline mutations in MYH by dHPLC and direct genomic DNA sequencing. Overall, 7 (8.9%) biallelic and 9 (11.4%) monoallelic MYH germline mutation carriers were identified. Among patients with a family history compatible with autosomal recessive inheritance (n=45), 1 (10.0%) out of 10 classical polyposis and 6 (17.1%) out of 35 attenuated polyposis patients carried biallelic MYH alterations, 2 of which represent novel gene variants (p.R171Q and p.R231H). Colorectal cancer was significantly (p<0.007) more frequent in biallelic mutation carriers (71.4%) compared with that of monoallelic and MYH mutation-negative polyposis patients (0 and 13.8%, respectively). On the basis of our findings and earlier reports, MYH mutation screening should be considered if all of the following criteria are fulfilled: (i) presence of classical or attenuated polyposis coli, (ii) absence of a pathogenic APC mutation, and (iii) a family history compatible with an autosomal recessive mode of inheritance.