Purpose: Taxane-containing combinations are recommended for the first-line therapy of advanced gastric cancer. It is not known which chemotherapy regimen is the best with trastuzumab for HER2-positive patients. The aim of this study was to compare taxane-containing intensified chemotherapy versus standard chemotherapy in combination with trastuzumab in the first-line treatment of HER2-positive advanced gastric adenocarcinoma. Methods: This study is a retrospective multicenter study of the Turkish Oncology Group. A total of 130 HER2-positive patients with inoperable locally advanced, recurrent, or metastatic gastric adenocarcinoma being given chemotherapy plus trastuzumab as the first-line treatment were included from 16 different oncology centers. Trastuzumab combination with intensified chemotherapy including taxane or standard chemotherapy was compared in terms of progression-free survival (PFS), overall survival (OS), and toxicity. Results: There were 108 patients in the standard and 22 patients in the intensified chemotherapy group. PFS of the standard and intensified group were 5.6 months (95% confidence interval [CI] 4.8-6.4) and 5.3 months (95% CI 2.6-8), respectively (p = 0.70). OS of the standard and intensified group were 11.1 months (95% CI 8.3-13.9) and 15.2 months (95% CI 12.7-17.7), respectively (p = 0.03). Repeated analysis excluding patients given any previous therapy revealed similar results. The intensified group had more fever and febrile neutropenia. Conclusion: Trastuzumab combination with intensified chemotherapy provides better OS in first-line treatment of HER2-positive advanced gastric cancer. Further large-scale studies should be performed in HER2-positive patients.