In order to ascertain whether genetic alterations occur during the early stages of gastric carcinogenesis, abnormal accumulation of p53 protein and mutation of its gene in stomach tissue showing intestinal metaplasia were investigated using immunohistochemistry and polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. Immunohistochemistry detected 19 foci showing nuclear accumulation of p53 protein in non-neoplastic gastric mucosa in a total of 756 sections (477 of which contained intestinal metaplasia) from 16 resected stomachs containing gastric adenocarcinomas. Of these 19 p53-positive foci, 17 were diagnosed histologically as incomplete-type intestinal metaplasia and 2 as pseudopyloric glands in the regenerative mucosa. Furthermore, 14 such foci were detected in 6 patients with multiple gastric cancers. No correlation between high-iron diamine (HID)-positive sulfomucin production and p53-positive glands was observed. The DNAs were extracted selectively from these p53-positive metaplastic glands and examined for p53 mutations by PCR-SSCP analysis followed by direct sequencing. In only 10 lesions could exons; 5 to 8 be investigated completely, and of these, 4 were shown to possess p53 mutations, which were on exon 5 in 3 cases and on exon 7 in 1 case. These results indicate that irreversible genetic changes had already occurred in morphologically non-neoplastic gastric mucosa with intestinal metaplasia, and are consistent with the hypothesis that intestinal metaplasia, especially the incomplete type, may contain precursor lesions of gastric cancer.