The amplification and overexpression of the c-erbB-2 gene are considered to be implicated in the process of carcinogenesis of a variety of human tumors. The amplification and overexpression of c-erbB-2 were investigated in 48 surgically resected human gastric cancers by means of fluorescence in situ hybridization and immunohistochemistry. DNA ploidy was determined by flow cytometry. The c-erbB-2 amplification was demonstrated as a cluster of signals, suggesting homogeneously staining region (HSR), in three tumors (6.3%) accompanied by the overexpression of its protein. Such overexpression was detected in another tumor without amplification of the c-erbB-2 gene. All tumors with amplification and overexpression of c-erbB-2 were differentiated adenocarcinoma histologically, but only 10.3 and 13.8% of differentiated carcinomas showed amplification and over-expression of the c-erbB-2 gene, respectively. There was no relationship between the amplification and overexpression of c-erbB-2 and the depth of tumor invasion and lymph node involvement. Three of four cases with overexpression of c-erbB-2 were classified into DNA aneuploid tumor.