Limited data are available regarding patterns of chemotherapy receipt and treatment-related toxicities for older women receiving adjuvant trastuzumab-based therapy. We used surveillance, epidemiology and end results (SEER)-Medicare data to identify patients ≥66 years with stage I-III breast cancer treated during 2005-2009, who received trastuzumab-based therapy. We examined patterns of chemotherapy receipt, and using multivariable logistic regression, we examined associations of age and comorbidity with non-standard chemotherapy. In propensity-weighted cohorts of women receiving standard and non-standard trastuzumab-based therapy, we also examined rates of (1) hospital events during the first 6 months of chemotherapy and (2) short-term survival. Among 2,106 women, 29.7 % were aged ≥76 and 66 % had a comorbidity score = 0. Overall, 31.3 % of women received non-standard chemotherapy. Compared to patients aged 66-70, older patients more often received non-standard chemotherapy [adjusted odds ratio (OR) = 4.1, 95 % confidence interval (CI) = 3.40-4.92 (ages 76-80); OR = 15.3, 95 %CI = 9.92-23.67 (age ≥ 80)]. However, comorbidity was not associated with receipt of non-standard chemotherapy. After propensity score adjustment, hospitalizations were more frequent in the standard (vs. non-standard) group (adjusted OR = 1.7, 95 % CI = 1.29-2.24). With a median follow-up of 2.8 years, 276 deaths occurred; the adjusted hazard ratio (HR) for death was lower in standard versus non-standard treated women (HR = 0.69, 95 % CI = 0.52-0.91). Among a population-based cohort of older women receiving trastuzumab, nearly one-third received non-standard chemotherapy, with the highest rates among the oldest women. Non-standard chemotherapy was associated with fewer toxicity-related hospitalizations but worse survival. Further exploration of treatment toxicities and outcomes for older women with HER2-positive breast cancer is warranted.