Four cases of colorectal polyps with epithelial serrated proliferation (CP-ESP) with malignant transformation were studied. In CP-ESP adjacent to carcinoma, if the nuclear size in the surface layer was significantly smaller than those in the bottom and the middle layers of the crypts, the specimen was defined as zone formation positive. If there was no significant difference among the layers, the specimen was defined as zone formation negative. Cell kinetics were evaluated using Ki-67 immunostaining. The CP-ESP regions of cases 1 and 2 showed zone formation with inferior and lateral glandular branching, and were qualitatively hyperplastic on cell kinetics. Cases 3 and 4 showed inferior and lateral glandular branching with no zone formation, and were kinetically neoplastic (adenoma). The histogenesis of hyperplastic polyps with atypia (cases 1 and 2) involves the hyperplastic polyp-carcinoma sequence. In contrast, the development of tubulovillous adenoma or serrated adenoma (cases 3 and 4) may involve the tubulovillous adenoma-carcinoma or serrated adenoma-carcinoma sequence.

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