We investigated the histogenesis of hyperplastic polyps of the stomach, in terms of cellular proliferation, by studying endoscopically removed and gastrectomized human gastric polyps either labeled with bromodeoxyuridine (BrdU) by in vitro flash labeling techniques or labeled in an isolated organ circulation system, in both of which, perfluorochemical artificial blood was employed. Immunohistochemistry with antibodies against BrdU and proliferating cell nuclear antigen (PCNA) was simultaneously employed. The generative cell zone of pedunculated and semipedunculated polyps was markedly expanded compared with that of the background mucosa, and this change also appeared in sessile polyps, although to a lesser degree. Enhanced proliferative activity was observed in both epithelial and stromal cells in areas of erosion. Our results demonstrate that the initial change in the histogenesis of hyperplastic polyps is an expansion of the generative cell zone, followed by interstitial edema and stromal cell proliferation, and that erosion can facilitate these changes. No correlation was found between the size of the polyps and the labeling indices. This finding explains, in part, the diversity of chronological changes in the size and shape of hyperplastic polyps.