A monoclonal antibody to PCNA, which can be used on routinely processed tissue, was applied to 25 cases of gastric adenomas and 64 cases of gastric adenocarcinomas in order to diffentiate adenoma and adenocarcinoma and also to evaluate the prognostic value in adenocarcinoma. The results were summerized as follows: The PCNA labelling index was 29.14+/-12.77% in control, 44.09+/-17.11% in adenoma and 80.15+/-10. 69 in adenocarcinoma, resulting in significant increase in adenocarcinoma compared to adenoma. In adenocarcinoma, no significant correlation was observed between PCNA labelling index and histologic grade, and there -was increased tendency of PCNA labelling index in proportion to depth of invasion without statistical significance. The PCNA index was significantly increased in advanced adenocarcinoma compared to early gastric carcinoma, and also in positive nodal metastasis group than in negative group. From above results, the PCNA stain will be able to provide a helpful method for the differential diagnosis between gastric adenoma and adenocarcinoma, and could be a useful prognostic factor in adenocarcinoma if other factors are considered together.