While reviewing the histological sections of 319 flat colorectal polyps at three different Hospitals in Tokyo, 7 cases of flat serrated adenomas were found. Flat serrated adenomas differ histologically from flat tubular adenomas. In the former the dysplastic cells are found initially at the lower part of the crypts of Lieberkuhn, and the epithelium of the sides of those crypts have serrated infoldings without dysplastic cells. In flat tubular adenomas, however, the dysplastic cells are initially found in the upper part of straight crypts (i.e. without epithelial infoldings). Six of the 7 serrated adenomas had low grade dysplasia (LGD) and the remaining one high grade dysplasia (HGD). Atypical mitosis were present in the dysplastic epithelium of serrated adenomas. Depending upon the topographic distribution of the dysplastic epithelium within the crypts flat serrated adenomas were divided into Type I, where the dysplastic epithelium was limited to the lower half of the serrated crypts, and Type II where the dysplastic epithelium was present even in the superficial half of the serrated crypts. Of the 7 serrated adenomas, 3 were Type I and the remaining 4 Type II lesions. The dysplastic epithelium seemed to have originated at the base of the crypts and progressed upwards, replacing the scalloped, serrated epithelium. Flat serrated adenomas appeared as benign lesions upon endoscopy (their pit pattern being identical to flat hyperplastic polyps), but in a previous survey of Swedish patients 3 of 47 flat serrated adenomas had invasive adenocarcinoma (i.e. with submucosal extension). Whether serrated adenomas evolved into invasive carcinoma in the Japanese should be assessed in a larger series of cases.