PURPOSE The purpose of this study was to determine whether trastuzumab improves prognosis of women with metastatic human epidermal growth factor receptor 2 (HER2)/neu -positive breast cancer beyond that of women with HER2/neu-negative disease. PATIENTS AND METHODS Two thousand ninety-one women with metastatic breast cancer diagnosed from 1991 to 2007, with known HER2/neu status and who had not received trastuzumab in the adjuvant setting, were identified. Disease was classified into the following three groups: HER2/neu negative, HER2/neu positive without first-line trastuzumab treatment, and HER2/neu positive with first-line trastuzumab treatment. Overall survival (OS) was estimated using the Kaplan-Meier product-limit method and compared between groups with the log-rank test. Cox proportional hazards models were used to determine associations between OS and HER2/neu status after controlling for patient characteristics. Results One hundred eighteen patients (5.6%) had HER2/neu-positive disease without trastuzumab treatment, 191 (9.1%) had HER2/neu-positive disease and received trastuzumab treatment, and 1,782 (85.3%) had HER2/neu-negative disease. Median-follow-up was 16.9 months. One-year survival rates among patients with HER2/neu-negative disease, HER2/neu-positive disease and trastuzumab treatment, and HER2/neu-positive disease and no trastuzumab treatment were 75.1% (95% CI, 72.9% to 77.2%), 86.6% (95% CI, 80.8% to 90.8%), and 70.2% (95% CI, 60.3% to 78.1%), respectively. In a multivariable model, women with HER2/neu-positive disease who received trastuzumab had a 44% reduction in the risk of death compared with women with HER2/neu-negative disease (hazard ratio [HR] = 0.56; 95% CI, 0.45 to 0.69; P < .0001). This HR varied with time and was significant for the first 24 months and not significant after 24 months. CONCLUSION Our results show that women with HER2/neu-positive disease who received trastuzumab had improved prognosis compared with women with HER2/neu-negative disease.