This parallel, randomized, open-label, multi-centre study will evaluate the effect on overall survival of trastuzumab (Herceptin) in combination with a chemotherapy compared to the chemotherapy alone in patients with HER2-positive advanced gastric cancer. Trastuzumab (Herceptin) will be administered as intravenous infusion of 6 mg/kg (loading dose 8 mg/kg) every 3 weeks. The chemotherapy consists of a combination of 6 cycles of fluorouracil (800 mg/m2/day intravenous infusion every 3 weeks) and cisplatin (80 mg/m2 intravenous infusion every 3 weeks), or capecitabine (Xeloda, 1000 mg/m2 po twice daily for 14 days every 3 weeks) and cisplatin (80 mg/m2 intravenous infusion every 3 weeks). Treatment with trastuzumab (Herceptin) will continue until disease progression. The target sample size is 300-600 patients.