A pivotal, randomized, multicenter, phase III trial was conducted to compare chemotherapy in combination with trastuzumab (Herceptin) vs. chemotherapy (anthracycline plus cyclophosphamide [AC] or paclitaxel) alone as first-line treatment for HER2-positive metastatic breast cancer. Results from a total of 469 patients, randomized to receive either chemotherapy alone or chemotherapy plus trastuzumab, revealed that the addition of trastuzumab improved time to disease progression significantly (7.6 vs. 4.6 months. P = 0.0001) compared with chemotherapy alone. The increase was higher in the trastuzumab plus paclitaxel subgroup (6.9 vs. 3.0 months, P = 0.0001) than in the trastuzumab plus AC subgroup (8.1 vs. 6.1 months. P = 0.0003). Patients receiving combination therapy also had a greater overall response rate (49% vs. 32%, P = 0.0002) and a longer median response duration (9.3 vs. 5.9 months, P = 0.0001) than those who received chemotherapy alone. Most importantly, median follow-up of 29 months revealed a significantly increased median survival in patients receiving trastuzumab plus chemotherapy (25.4 vs. 20.3 months, P < 0.025) compared with those receiving chemotherapy alone. Trastuzumab plus chemotherapy was well tolerated; adverse events were typically mild-to-moderate chills and fever and occurred in approximately 40% of patients, primarily following the first administration only.