We have identified that HER-2/neu-derived peptides are naturally processed as tumor rejection antigens recognized by tumor-specific, HLA-A2-restricted cytotoxic T lymphocytes in gastric cancer. To evaluate candidates for immunotherapy using HER-2/neu-derived, HLA-A2-restricted peptides, we examined the frequency of HLA-A2 relating to HER-2/neu overexpression or the infiltrating grade of tumorinfiltrating lymphocytes (TILs) in Japanese patients with gastric cancer. HER-2/neu-overexpressing tumors detected by immunohistochemistry amounted to 19% of primary gastric cancers and HLA-A2-positive patients with gastric cancer were 31% of primary gastric-cancer cases. Finally, gastriccancer patients with both HLA-A2-positive and HER-2/neuoverexpressing tumors amounted to 6.6% of these cases. There was no significant difference in the infiltrating grade of TILs between gastric cancers overexpressing HER-2/neu and those that did not. Gastric cancer is 1 of the most common cancers occurring in Japan today. The control of gastric cancer at advanced stages still remains difficult, despite various treatments such as gastrectomy with extensive lymphadenectomy 1 and surgery combined with chemotherapy. 2 We and others reported that MHC class I-restricted cytotoxic-T lymphocytes (CTLs) from gastric-cancer patients can react specifically against autologous tumor cells. [3][4][5][6] Adoptive immunotherapy based on gastric cancer-specific CTLs, or vaccinations based on the MHC class I-associated tumor antigens that they recognize, represent alternative therapeutic approaches. The presence of tumor-associated antigens recognized by gastric-cancer-specific CTLs restricted by HLA-A31 6 or HLA-A2.1 molecules 4 has been demonstrated, although the nature of these antigens has not been defined. Moreover, we have identified that HER-2/neu-derived peptides are naturally processed as tumorassociated antigens recognized by tumor-specific, HLA-A2-restricted CTLs in gastric cancer. 7 Thus, HER-2/neu-derived peptides may be considered as candidates for tumor-specific immunotherapy such as tumor vaccination in patients with gastric cancer.The HER-2/neu protooncogene encodes a 185-kDa transmembrane glycoprotein with tyrosine-specific kinase activity. 8 The HER-2/neu protooncogene is amplified and overexpressed in approximately 30% of human ovarian and breast tumors 9 and in 20% of gastric cancers. 10 HLA-A2-restricted CTL epitopes derived from HER2/neu recognized by ovarian-11,12 and breast-13 cancerspecific CTLs have previously been defined, in addition to those specific for gastric cancer. The identification of several new HLA-A2-restricted CTL epitopes from HER-2/neu that can activate CTLs from healthy donors and patients with advanced ovarian carcinoma has also been reported. 14,15 These results indicate that HER-2/neu-derived peptides could be broad targets for immunotherapy against these types of cancers. It is important to evaluate candidates for tumor vaccination with HER-2/neu-derived peptides in patients with gastric cancer. Because it has...