The aim of this study was to evaluate the pattern of pS2 protein expression in premalignant and malignant lesions of gastric epithelium. We analysed, by immunohistochemistry, the pS2 expression in six samples of normal gastric mucosa, 18 cases of chronic atrophic gastritis with intestinal metaplasia (IM), 10 hyperplastic polyps, 11 adenomatous polyps and 50 gastric carcinomas, together with the respective samples of adjacent non-neoplastic mucosa. pS2 is expressed throughout foveolar and superficial epithelium of normal gastric mucosa and this pattern is retained in chronic atrophic gastritis out of IM lesions. pS2 expression is confined to goblet cells in complete IM and occurs both in goblet and columnar cells in incomplete IM. Hyperplastic polyps displayed significantly higher pS2 expression than adenomatous polyps. In gastric carcinomas, pS2 expression was observed in 66.0% of cases, being significantly higher in diffuse (88.9%) than intestinal type carcinomas (53.6%). A subset of carcinomas of the latter group displayed pS2 immunoreactivity in a high percentage of cells with a pattern similar to that of hyperplastic polyps. Our results demonstrate there are major changes in pS2 expression, which can be used as a marker of gastric-type differentiation during the process of gastric carcinogenesis, and support the existence of at least two pathways of malignant transformation of gastric mucosa: one via intestinal metaplasia and adenomatous dysplasia, leading to glandular carcinomas with intestinal-type differentiation, the other via hyperplastic changes or de novo changes, leading to diffuse carcinomas and to a subset of glandular carcinomas displaying gastric-type differentiation.