Colorectal adenomas are histologically classified into two categories: tubular and villous adenomas. A mixed (tubulovillous) phenotype has also been recognized. Although two other histologic phenotypes, serrated and microtubular adenomas, were reported later, they have not yet been included in any of the larger series reported in the literature. In this study, we classified all colorectal adenomas registered at this department between 1993 and 2000 (n = 3135) into five histologic categories: tubular (66%; n = 2074), villous (9%; n = 281), mixed (18%; n = 551), serrated (6%; n = 202), and microtubular (1%; n = 27) . More than 80% of the tubular, villous, serrated and microtubular adenomas were located in the colon. None of the microtubular adenomas was found in the rectum. Cell proliferation in tubular and villous adenomas occurred initially in the upper part of the dysplastic glands, in serrated adenomas in the lower part of the dysplastic crypts, and in microtubular adenomas in the deeper part of the dysplastic "rings". Because of these distinctive morphologic features and cell-proliferative characteristics, serrated and microtubular adenomas should be regarded as independent phenotypes. To compare the frequency of these adenoma phenotypes, we suggest that serrated and microtubular adenomas be included in future reports on the histologic characteristics of colorectal polyps.