In a cytogenetic analysis of 9 gastric and lower esophageal adenocarcinomas, we detected nonrandom rearrangements involving the region 11p13-15 in 8, thus identifying for the first time a specific chromosomal lesion in these tumors. In addition, rearrangements involving 3p21, translocations among the D group chromosomes, and i(5p) were each observed in more than half of the cases. The overall pattern of aberrations encountered in adenocarcinomas of gastric and lower esophageal origin was similar, suggesting that the tumors arising at these anatomical sites are biologically related. We also encountered cytogenetic evidence for gene amplification in the form of homogeneously staining regions and double-minute chromosomes in primary as well as metastatic lesions, which is consistent with amplification of a number of cellular oncogenes in these tumors detected by others and us at the molecular level. These cytogenetic findings are discussed in relation to nonrandom chromosome abnormalities and gene amplification reported in other types of adenocarcinoma.