Adenomas are established pre-malignant lesions in colorectal carcinogenesis. To date the adenoma-carcinoma sequence for the development of colorectal carcinoma (CRC) has been based largely on molecular data of exophytic, polypoid-type adenomas. Subsequently, a different type of adenoma has been identified: the flat adenoma, so called for its flat, non-exophytic appearance, making it less likely to be detected during conventional endoscopy. However, due to technological advances in endoscopic methods, flat-type adenomas can now frequently be detected and are no longer considered rare colorectal lesions. The phenotype of flat colorectal adenomas differs macroscopically and histologically from exophytic adenomas. Flat colorectal adenomas, as a rule, are tubular structures often revealing high-grade dysplasia, irrespective of the size or villous component. Flat adenomas have also been recognised as pre-cancerous lesions in gastric cancer. Unlike the wealth of clinical and molecular information available for polypoid (exophytic) adenomas, molecular profiles of flat-type lesions have not yet been characterised systematically and the full clinical significance hereto realised. Previous molecular investigation of the K-ras gene in flat colorectal adenomas suggests a distinct pathway in their development. In this study, mutation analysis of the adenomatous polyposis coli (APC) gene using the protein truncation test (PTT) in 20 flat colorectal adenomas in a selected group of 16 patients without hereditary predisposition to colorectal cancer, revealed double truncations of the APC gene in four adenomas. In one of these adenomas a third mutation was detected by DNA sequence analysis.