Aim: The aim of this study is to verify the usefulness of endoscopic mucosal resection (EMR) technique for the treatment of large flat adenomas of the colorectum. Material and Method: We have treated 20,982 colonic neoplasms including benign adenomas and early cancers, among which 787 lesions were large (more than 10 mm in diameter) flat adenomas. They were divided into two groups; granular and non-granular. The former was subdivided into homogeneous and nodular mixed, and the latter was further divided into pseudo-depressed and flat-elevated. The pit patterns, or microstructures of the surface of the lesions were classified into I, II, IIIs, IIIL, IV, VI and VN. Results: There were no major complications requiring surgical operation, related to the EMR treatment. The respective rates of invasive cancer in homogeneous granular, nodular mixed, pseudo-depressed, and flat-elevated group were 0.8%, 23.2%, 18.5% and 9.0%. The invasive rate of homogeneous granular type was 0% in lesions 20-29 mm and 4.3% in those more than 30 mm in size, but that of pseudo-depressed type was 42.1% in lesions 20-29 mm. The pit patterns on magnifying colonoscopy correlated well with the final histological diagnosis; nearly one hundred percent of type IIIL or IV pit pattern represented benign adenoma, while more than eighty percent of the lesions with type VN pit pattern were invasive cancers. Conclusion: Large flat adenomas are usually not so invasive compared with the size, and therefore are good candidates of EMR treatment. But careful decision is required for treating some subgroups of them, nodular-mixed type and pseudo-depressed type, because they are more invasive than others. Pit pattern analysis by magnifying colonoscopy is useful to predict the histology.