Background: Few data are available about factors associated with completion of adjuvant trastuzumab in older women with BC. We examined rates and predictors of adjuvant trastuzumab completion for older women with early-stage BC. Methods: We used Surveillance, Epidemiology, and End Results (SEER)-Medicare data to identify 1,319 patients ≥66 years with early-stage BC diagnosed between 1999-2007 who received trastuzumab. Completion of trastuzumab was defined as >270 days of therapy. We examined patient, clinical and geographic characteristics associated with trastuzumab completion using multivariable logistic regression. We also assessed rates of hospital admissions for cardiac events during treatment. Results: Most of the 1,319 women were aged ≤76 (70%) and had a comorbidity score=0 (88%); 37% and 23% received anthracyline-taxane-based and taxane-based therapy, respectively, and 16% received trastuzumab without chemotherapy. Overall, 982 women (74.5%) completed trastuzumab. Factors associated with completion are shown below. During treatment, 56 patients (4.2%) had 65 hospital admissions for cardiac events (3.0% in those who completed trastuzumab versus 8.0% in those who did not, p<.001). Conclusions: One-quarter of older patients who initiated adjuvant trastuzumab did not complete therapy. Older women, Hispanic women, those with more comorbidity, and those receiving anthracycline-taxane-based chemotherapy all had lower odds of completion. Rates of hosptializations for cardiac events were higher in those who did not complete therapy.