BACKGROUND Gastrin hormone is trophic to in vitro gastric cancer, and the antigastrin antibodies (AGAs) are antiproliferative and antimetastatic. Human gastric cancers overexpress gastrin genes and receptors that react to gastrin's trophic effects. Immunogen G17DT elicits a specific and high‐affinity AGA. The authors evaluated G17DT vaccination given with cisplatin plus 5‐fluorouracil for the treatment gastric adenocarcinoma. METHODS In this multicenter, Phase II study, patients received G17DT vaccination intramuscularly on Weeks 1, 5, 9 and 25 and cisplatin plus 5‐fluorouracil every 28 days. Eligible patients had untreated, metastatic, or unresectable gastric or gastroesophageal adenocarcinoma with near‐normal organ function. The primary endpoint of the study was the over response rate (ORR), and secondary endpoints included overall survival (OS), safety, and the impact of successful vaccination on patient outcome. RESULTS In total, 103 patients were enrolled in 5 countries. Seven patients who were overdosed inadvertently with 5‐fluorouracil (a major protocol violation) were removed from the analysis. The confirmed ORR was 30% in 79 patients who were evaluated for response. The median time‐to‐progression (TTP) was 5.4 months, and the median survival (MS) was 9.0 months (n = 96 patients). Sixty‐five of 94 patients who were vaccinated (69%) had 2 consecutive AGA titers of ≥1 units (successfully vaccinated patients or immune‐responders). The TTP was longer in immune‐responders than in immune‐nonresponders (P = .0005). Similarly, the MS was longer in immune‐responders than in immune‐nonresponders (10.3 months vs. 3.8 months; P ≤.0001). In a multivariate analysis, successful vaccination was an independent OS prognosticator (P = .0001). G17DT did not have an adverse effect on safety. CONCLUSIONS The results demonstrated that successful G17DT vaccination was correlated with longer TTP and MS. AGA response was an independent OS prognosticator. A Phase III evaluation of G17DT in gastric cancer is warranted. Cancer 2006. © 2006 American Cancer Society. The authors evaluated a G17DT vaccination that was given with combined cisplatin and 5‐fluorouracil for the treatment gastric adenocarcinoma. The results demonstrated that successful G17DT vaccination was correlated with longer time to disease progression and median survival and that antigastrin antibody response was an independent predictor of overall survival.