Microscopic examination of the adenocarcinoma arising in polyp by hematoxylin-eosin (H & E) and immunohistochemical stainings. (H & E) Small adenocarcinoma areas in the hyperplastic polyp show irregular and fused tubular structures, associated with surface erosion, granulation, hemorrhage, and inflammatory infiltrate. These structurally atypical epithelial cells contain mildly to focal moderately enlarged nuclei and occasionally prominent nucleoli with loss of cellular polarity. (CA19-9) These atypical glands are immunohistochemically positive for CA19-9, associated with strong expression especially in the secreted mucin. The CA19-9-positive staining displays not only a cytoplasmic but also an apical pattern. (p53) Distinct nuclear staining for p53 is occasionally conforming to these atypical cells (arrows). (Ki-67) Moreover, Ki-67 (MIB-1) labeling index is high (more than 5% compared to hyperplastic polyps), conforming to the small adenocarcinoma areas.