OBJECTIVES: The NSABP/NCCTG trial (Romond et al. NEJM 2005;353:1673-1684) established the efficacy of trastuzumab in the adjuvant treatment of HER2+ early stage breast cancer (HER2+BC). Yet, little is known about the patterns of use and outcomes of adjuvant trastuzumab in clinical practice. The study aimed to estimate the overall survival (OS) and relapse-free survival (RFS) of HER2+BC patients treated with adjuvant trastuzumab in the US Department of Defense (DOD) practice setting. METHODS: Adult women initiating adjuvant trastuzumab within 1 year of BC surgery were identified in the DOD health claims database (01/2003-12/2012). An algorithm based on secondary neoplasm ICD9 codes and treatment gaps and initiations was used to identify relapses. OS and RFS unadjusted rates at 3 and 4 years after the initiation of the adjuvant trastuzumab treatment were estimated from Kaplan-Meier plots. RESULTS: The study sample included 3,188 women (median age 63 years), followed for a median of 3.3 years after the initiation of trastuzumab and treated continuously with trastuzumab for a median of 12 months. Of these 3,188 women, 13.8% received neo-adjuvant therapy prior to the surgery, 17.7% relapsed, and 7.9% died during the follow-up. The OS rates at 3 and 4 years were 93.2% (95% CI 92.1%-94.2%) and 90.0% (88.6%-91.2%), respectively. The corresponding RFS rates were 78.8% (77.1%-80.3%) and 75.8% (74.0%-77.5%), respectively. CONCLUSIONS: The findings suggest that most HER2+BC patients in the DOD practice setting received per-label trastuzumab treatment (for 52 weeks) and had OS rates that are similar to the OS rates that were previously observed in the NSABP/NCCTG clinical trial (90.0% vs. 93% at four years). The lower RFS rates observed in this study versus the NSABP/NCCTG trial (75.8% vs. 85.7% at 4 years), may be partially explained by differences in the characteristics of the patients, including age.