The two most common inherited forms of colorectal cancer are familial adenomatous polyposis and hereditary non-polyposis colorectal cancer. Simultaneous inheritance of both an APC gene mutation and a mismatch repair gene (for example, MLH1) mutation has never been described. In the present case report, we report rapidly progressive adenomatous polyposis in a 10 year old boy with a germline frame shift mutation in the APC gene and a germline splice site mutation in the MLH1 gene. Immunohistochemical investigations showed abnormal expression of beta-catenin in early adenomas with low grade dysplasia, attributed to the APC gene mutation. Subsequent loss of function of the MLH1 gene, as shown by absent immunostaining of its protein in adenomas with high grade dysplasia, may well have caused the rapid progression to high grade dysplasia in many of the adenomas.