Purpose: To evaluate the efficacy of adding lapatinib to capecitabine and oxaliplatin (CapeOx) in patients with previously untreated human epidermal growth factor receptor 2 (HER2) -amplified advanced gastroesophageal adenocarcinoma. Patients and Methods: Patients with HER2-positive advanced gastroesophageal adenocarcinoma were randomly assigned at a one-to-one ratio to CapeOx plus lapatinib 1,250mg or placebo daily. Primary end point was overall survival (OS) in patientswith centrally confirmed HER2 amplification in the primary efficacy population. Results: A total of 545 patients were randomly assigned, and 487 patients comprised the primary efficacy population. Median OS in the lapatinib and placebo arms was 12.2 (95% CI, 10.6 to 14.2) and 10.5 months (95% CI, 9.0 to 11.3), respectively, which was not significantly different (hazard ratio, 0.91; 95% CI, 0.73 to 1.12). Median progression-free survival in the lapatinib and placebo arms was 6.0 (95% CI, 5.6 to 7.0) and 5.4 months (95% CI, 4.4 to 5.7), respectively (hazard ratio, 0.82; 95% CI, 0.68 to 1.00; P = .0381). Response rate was significantly higher in the lapatinib arm: 53% (95% CI, 46.4 to 58.8) compared with 39% (95% CI, 32.9 to 45.3) in the placebo arm (P = .0031). Preplanned exploratory subgroup analyses showed OS in the lapatinib arm was prolonged in Asian and younger patients. No correlation was observed between HER2 immunohistochemistry status and survival. There were increased toxicities in the lapatinib arm, particularly diarrhea. Conclusion: Addition of lapatinib to CapeOx did not increase OS in patients with HER2-amplified gastroesophageal adenocarcinoma. There were clear differences in the effect of lapatinib depending on region and age. Future studies could examine this correlation.