Four hundred and seventy-seven hyperplastic polyps, removed by endoscopic polypectomy, were reviewed with special reference to the appearance of dysplastic foci and their malignant transformation. Focal carcinomas were found in 10 hyperplastic polyps, which corresponded to 2.1% of the total of 477 hyperplastic polyps. The location of cancer was at the head or at the surface of the polyp, and was intramucosal. Dysplastic foci equivalent in the degree of histological atypia to adenoma were found adjacent to the cancerous foci in these 10 polyps. Dysplastic foci were also found in 19 hyperplastic polyps without cancerous foci, which corresponded to 4.0% of the total hyperplastic polyps. In connection with the histogenesis of malignant transformation of hyperplastic polyps, the results of clinicopathological examination and immunohistochemical stains strongly suggested that cancer cells arose from the dysplastic area in hyperplastic polyps, rather than directly from nondysplastic hyperplastic epithelium.