Background: We examined guideline-concordant care among women with HER2+ MBC and determined the magnitude of differences in guideline-concordant care between those with positive and negative hormone receptor (HR) status by utilizing a non-linear decomposition technique. Methods: We conducted a retrospective observational cohort study using the Surveillance, Epidemiology, End Results-Medicare linked database. The study cohort consisted of women age >66 years diagnosed with HER2+ MBC in 2010–2013 (N=241). Guideline-concordant initial care within 6 months of cancer diagnosis was defined as per NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines). A multivariate logistic regression was performed to identify the significant predictors of guideline-concordant care. A post-regression non-linear decomposition was conducted to examine the magnitude of disparities in guideline concordant care by women’s HR status. Results: 76.8% of the study cohort received guideline-concordant care, while 23.2% did not. As compared to those who did not receive guideline-concordant care, women who received guideline-concordant care were significantly more likely to have positive HR status (adjusted odds ratio (AOR)=2.11; P=.04), had good performance status (AOR=3.46; P=.0008), and had a higher number of oncology visits (AOR=8.05; P<.0001). With 1 year increase in age at cancer diagnosis, there was 5% lesser likelihood of receiving guideline-concordant care (AOR=0.95; P=.04). From the decomposition analysis, 19.0% of the disparity in guideline-concordant care between women with positive and negative HR status was explained by differences in their characteristics. Enabling characteristics (marital status, census-level income, and education) explained the highest (22.8%) proportion of the disparity, followed by external environmental factors (location of residence, SEER region, hospitals offering oncology services) at 5.3%, and need-related factors (tumor grade, comorbidity, performance status, number of metastases) at 3.2%. Conclusion: Almost one quarter of the study cohort did not receive guideline-concordant care. There are opportunities to improve cancer care for women with negative HR status who have lower socioeconomic status. The high unexplained portion of differences in guideline-concordant care (81.0%) can be due to patient preferences for treatment, propensity to seek care, and organizational and physician-level factors not captured in the database.