Prognostic significance of c-erbB-2 gene abnormalities is unclear in the poorly differentiated type of gastric carcinoma, because the abnormalities of this gene have been reported to be restricted to the differentiated type of gastric carcinoma. In this study, correlation of c-erbB-2 gene amplification/overexpression of mRNA and protein were studied in the poorly differentiated type of gastric carcinoma. c-erbB-2 gene amplification determined by the slot-blot hybridization was observed in 11 (13%) of 82 gastric cancer, and 8 of 11 tumors were poorly differentiated. In addition, c-erbB-2 mRNA expression was studied by the reverse transcriptase-polymerase chain reaction. Four (17%) of 24 tumors showed overexpression of c-erbB-2 mRNA, and all these four exhibited morphologically a poorly differentiated type. Among 157 poorly differentiated gastric cancers, 20 (13%) tumors showed immunohistochemically c-erbB-2 protein expression. These tumors had significantly higher incidences of larger tumor, serosal invasion-positive tumors, node-positive tumor, or peritoneal dissemination-positive tumor than those without c-erbB-2 expression. Furthermore, patients with c-erbB-2 protein overexpression ran poorer prognoses than those without c-erbB-2 expression. From these results, we conclude that expression c-erbB-2 tissue status may be a good prognostic indicator in poorly differentiated gastric carcinoma.