Background: Trastuzumab has been approved for treatment of patients with HER2-positive metastatic gastric carcinoma; however, relatively little is known about the role of HER2 in the natural history of this disease. Methods: Patients enrolled in the INT-0116/SWOG9008 phase III gastric cancer clinical trial (n=559) were retrospectively evaluated for HER2 gene amplification by fluorescence in situ hybridization (FISH)(n=258), overexpression by immunohistochemistry (IHC)(n=148) and HER2 amplification by silver-enhanced in situ hybridization (n=77) based on availability of tissue specimens. The purpose of the original clinical trial was to evaluate the benefit of post-operative 5-fluorouracil/leucovorin plus radiation therapy compared to surgery alone. Results: HER2 gene amplification rate by FISH was 10.9% in tumor tissue from 258 patients evaluated. HER2 status determined by FISH was 92% concordant with SISH. HER2 overexpression rate by IHC was 12.2% among 148 patients evaluated, with 90% agreement between FISH and IHC. There was a significant interaction between HER2 status by FISH and treatment with respect to both OS (p=0.034) and DFS (p=0.020). Among patients with HER2 non-amplified cancers, treated patients had a median OS of 44 months compared to 24 months for patients in the surgery only arm (34 and 17 months respectively for DFS, p=0.003). Among 28 patients with HER2 amplified cancers, the medians for OS were 16 months in the treated arm, and 22 months in the surgery arm (p=0.55) (13 and 11 months respectively for DFS, p=0.87). We were unable to detect a statistically significant treatment benefit in this small subset of patients with HER2 amplification. HER2 amplification status was not an independent prognostic marker of OS among patients who received no postoperative chemotherapy or radiation therapy (p=0.76). Conclusions: Patients lacking HER2 amplification responded significantly to treatment as indicated by both OS and DFS.