Background: Trastuzumab was approved in the United States (US) in September 1998 for the treatment of HER2+ metastatic breast cancer (MBC). This model estimates the total number of life years saved (LYS) in US women treated with trastuzumab over a 15-year period (1999-2013). Methods: Using US population estimates and cancer registry-based incidence data, we estimated the number of women with recurrent stage I-III or de novo stage IV HER2+ MBC by year, age, hormone receptor, and nodal status. Trastuzumab utilization was based on published studies of HER2 testing rates, true positive rates in the community, and treatment initiation rates. Survival was estimated by extrapolating survival data pooled across 5 trials and 2 observational studies separately for women treated with trastuzumab and with chemotherapy alone. Few studies reported survival in women with HER2+ MBC without trastuzumab (N=3). Sensitivity analyses were conducted by estimating overall survival from the initial phase 3 trial (67% of placebo patients crossed over to trastuzumab after progression; HR=0.80), and assuming a higher risk reduction to account for crossover effects in clinical trials (HR=0.60). Results: In the base case, approximately 83,462 women with HER2+ MBC were estimated to receive 1^st line trastuzumab over a 15-year period. The pooled median overall survival across studies without and with trastuzumab was 21.2 and 35.5 months, respectively. Patients were projected to live a total of 216,290 life years if trastuzumab had not been available and if they received chemotherapy only. These same patients were estimated to live a total of 294,877 life years with first-line trastuzumab, for an incremental benefit of 78,587 LYS. In sensitivity analysis, total LYS ranged from 48,334-96,360. Conclusions: Real-world evidence supports a median overall survival of approximately 36 months in women with HER2+ MBC receiving 1^st line trastuzumab. Using a population-based, conservative model, we found that trastuzumab use has resulted in > 75,000 life years over 15 years in women with HER2+ MBC. Future research is warranted to examine the characteristics, experiences, and outcomes among women living longer with HER2+ MBC.