Examination of p53 mutation spectra may provide clues to molecular mechanisms involved in different histologic types of gastric cancer. A total of 105 gastric cancer cases classified according to the Laurén's system were selected from a high-risk area around Florence, Italy. Exons 5-8 of the p53 gene were examined for mutations by the polymerase chain reaction-single strand conformation polymorphism technique and DNA sequencing, using DNA from formalin-fixed paraffin-embedded tissues. Mutation frequency was similar in intestinal-type (12/28) and unclassified tumors (9/18), but was significantly lower in diffuse cancers (12/57, P < 0.05). A similar frequency of p53 mutations was observed among tumor stages in both intestinal-type and unclassified cancers, but in diffuse tumors mutations tended to be associated with invasion beyond the muscularis propria. When base changes were considered, G:C-->A:T transitions at CpG sites were the most common mutations for all the three tumor types with 6 of 11 (55%) in intestinal type, 8 of 12 (67%) in diffuse type, and 5 of 8 (63 %) in unclassified tumors. Frequent p53 mutations in both intestinal-type and unclassified tumors support the hypothesis that unclassified tumors represent variants of the intestinal type and suggest that unclassified tumors, like the intestinal type, may also associate with environmental exposures. The predominance of G:C-->A:T transitions at CpG sites, which are associated with methyltransferase-induced DNA methylation at carbon 5 of cytosine, in all three tumor types suggests that the status of DNA methylation may be the major determinant for p53 mutations and may be also equally important in gastric carcinogenesis regardless of histology.