Abstract Gastric carcinogenesis is attributable to interacting environmental and genetic factors, through a sequence of events including intestinal metaplasia. Using a fluorescence in situ hybridization technique, we investigated the occurrence of aneuploidies of chromosomes 3, 7, 8, 9, and 17, TP53 gene deletion, and expression of p53 in 21 intestinal metaplasia (IM) samples from cancer-free patients and in 20 gastric adenocarcinoma samples. Aneuploidies were found in 71% (15/21) of the IM samples. Trisomy of chromosomes 7 and 9 occurred mainly in complete-type IM; in the incomplete type, trisomy of chromosomes 7 and 8 were more commonly found. The TP53 gene deletion was observed in 60% (3/5) of the IM cases, and immunohistochemistry revealed p53 overexpression in 12% (2/17) of the analyzed IM cases. All gastric adenocarcinoma cases presented higher frequencies of trisomy or tetrasomy of chromosomes 3, 7, 8, 9, and 17. The TP53 deletion was found in all three of the gastric adenocarcinoma analyzed for it, and immunohistochemistry detected overexpression of protein p53 in 80% (12/15) of the analyzed cases. Our study revealed for the first time the presence of aneuploidies of chromosomes 7, 8, 9, and 17 and of TP53 gene deletion and overexpression in IM samples from cancer-free patients. These results suggest that IM and gastric adenocarcinoma may share the same genetic alterations.