Familial adenomatous polyposis (FAP) and attenuated FAP are autosomal dominant disorders characterised by multiple colorectal adenomas and cancers. Both are caused by inherited mutations in the APC gene, and management includes genetic testing, colonoscopic surveillance, and prophylactic surgery for the relatives of index cases. Among 614 families recorded in six regional registers of polyposis in the UK, we identified 111 with neither dominant transmission nor evidence of APC mutation. Molecular genetic analysis showed that 25 had biallelic mutations of the MYH gene. Since our data show that MYH polyposis can be transmitted as an autosomal recessive trait, a change in genetic counselling, testing, and surveillance is needed.