A proportion of gastrointestinal neuroendocrine tumors are aggressive; however, little is known of molecular determinants of their growth, and molecular studies have identified no useful prognostic factors. Overexpression of HER-2/neu is common in some nonendocrine tumors, frequently correlates with increased tumor aggressiveness, and can be used as a basis of treatment with trastuzumab. Little is known of its expression in malignant pancreatic endocrine tumors. In the present study HER-2/neu gene amplification and expression was determined in 43 gastrinomas from different patients. Results were correlated with clinical, laboratory, and tumor characteristics including tumor growth. HER-2/neu gene amplification was assessed by differential PCR, mRNA levels assessed by quantitative PCR, and protein by immunohistochemistry. Fourteen percent of patients had HER-2/neu gene amplification in tumors compared with levels in their WBCs. HER-2/neu mRNA varied over a 700-fold range. However, only 3% exceeded levels seen in normal pancreas, and immunohistochemistry did not show protein overexpression in any tumor (n = 10). HER-2/neu mRNA levels were significantly higher (P = 0.032) in tumors associated with liver metastases but not with tumor location or size. These results show that HER-2/neu amplification/overexpression does not seem to play a role in the molecular pathogenesis of most gastrinomas, as suggested in a previous study involving small numbers of cases. However, mild gene amplification occurs in a subset, and overexpression is associated with aggressiveness. Therefore, HER-2/neu levels could have prognostic significance as well as identify a patient subset with gastrinomas who might benefit from trastuzumab treatment.