Pancreatic ductal adenocarcinoma (PDAC) is the major pancreatic tumor and carries an extremely poor prognosis. Coexpression of epidermal growth factor receptor (EGFR) and the HER-2 oncoprotein has been reported to be related to the invasion and an adverse clinical outcome of human pancreatic ductal adenocarcinomas. HER-2 amplification, as determined by fluorescent in situ hybridization (FISH) analysis, has been identified as a positive predictor of response to EGFR tyrosine kinase inhibitor treatment in some other cancers. The aim of this study was to investigate the coexpression rate and amplification status of HER-2 oncogene in EGFR positive pancreatic ductal adenocarcinoma (PDAC) by immunohistochemistry and FISH. Overexpression of EGFR (>or=2+) was seen in 65% (21/32) of the study cases. EGFR gene amplification was not detected in any of the 32 PDACs. Overexpression of HER-2 protein (>or=2+) was seen in 17% (5/28) of the study cases and in 24% (5/21) of EGFR positive cases. None of the EGFR negative tumors showed HER-2 overexpression or gene amplification. The HER-2 gene locus was amplified in 11% (3/28) of the study cases and in 14% (3/21) of EGFR positive PDACs. There was 60% concurrence between HER-2 gene amplification and HER-2 protein expression in this study. These results suggest that HER-2 is an important cooperating member of the EGFR signal pathway in a subset of PDAC.