Trastuzumab (Herceptin®) is a humanized monoclonal antibody that binds selectively to human epidermal growth factor 2 (HER2), interfering with its downstream cancer-promoting effects. This article focuses on the efficacy and tolerability of trastuzumab in HER2-positive advanced gastric cancer. The potential of trastuzumab as a cytotoxic for use in gastric cancer was confirmed by in vitro studies in HER2-positive gastric cancer cell lines and gastric cancer xenograft models. In a randomized, controlled, open-label, multinational trial in patients with HER2-positive advanced gastric cancer, trastuzumab plus chemotherapy (cisplatin plus capecitabine or 5-fluorouracil) was significantly more efficacious than chemotherapy alone, in terms of a longer median overall survival (13.8 vs. 11.1 months in the chemotherapy alone group) [primary endpoint], a longer median progression-free survival, and a higher response rate. Trastuzumab was efficacious across patient subgroups, although stronger effects were observed in a subgroup with high HER2 overexpression (immunohistochemistry 2+/fluorescence in-situ hybridization positive or immunohistochemistry 3+). There was a slightly higher tolerability burden in the trastuzumab plus chemotherapy group than with chemotherapy alone, based on small between-group numerical differences in rates of common gastrointestinal and general adverse events. Most individual adverse events reported in this trial were at a grade 1 or 2 level of severity. However, in both treatment groups approximately half of the haematological adverse were at a grade 3 or 4 level of severity, with no marked between-group differences. Trastuzumab in combination with cisplatin and a fluoropyrimidine is an effective regimen for patients with HER2-positive advanced gastric cancer, has acceptable tolerability and represents an important advance in the treatment of gastric cancer.