Patients with gastric and gastroesophageal junction (GEJ) adenocarcinomas that are HER2 positive by immunohistochemistry (IHC) or in situ hybridization show a significant survival benefit with trastuzumab therapy. In situ hybridization is traditionally done by fluorescence in situ hybridization (FISH), despite some limitations. An alternative is the dual in situ hybridization (Dual ISH) technique that is fully automated and uses differentially labeled CEP17 and HER2 probes that can be read by light microscopy on 1 slide. The aim of this study was to assess the utility of Dual ISH in gastric/GEJ cancer and to compare the results with those obtained by IHC and FISH. Cases of gastric/GEJ adenocarcinoma were analyzed by IHC, FISH, and Dual ISH and the correlation between methods calculated. Results for 50 patients were available. There was a 98% (49/50) concordance rate between Dual ISH and FISH. One discrepant case was nonamplified by FISH but showed focal amplification by Dual ISH. Discrepancy was attributed to tumor heterogeneity, which was a frequent finding (78% of HER2-positive cases). There was excellent correlation between Dual ISH and FISH for assessment of HER2 amplification. Dual ISH was rapid, easy to interpret, and maintained cell morphology, which was valuable in identifying tumor heterogeneity.