Background: National Comprehensive Cancer Network breast cancer guidelines suggest sequencing of systemic therapy. This study examined sequencing of endocrine and chemotherapy treatments to better understand real-world treatment patterns for HR+/HER2- advanced breast cancer. Methods: A proprietary clinical cancer database with physician-reported clinical data on patients with breast cancer was linked to medical and pharmacy claims (2008-2012) from a national US health plan. Study patients had HR+ and HER2- status. Advanced cancer cohorts were defined: stage III (SIII) or IV (SIV) at initial diagnosis, or developed metastases following initial diagnosis (MET). Index date was the first date of advanced cancer diagnosis or date of metastases following initial diagnosis. Health plan enrollment for 3 months pre- and ≥ 3-months post- index date was required; patients who died within 3 months after index date and were continuously enrolled were retained. A 3-month baseline period assessed prior treatment; a variable follow-up (until disenrollment or 31 Oct 2012) assessed patterns of endocrine and chemotherapy treatments following index date. Results: A total of 954 breast cancer patients met study inclusion criteria and were HR+/HER2- with ≥ 3 months of continuous enrollment after index date, with 369 of the 954 (38.68%) SIII, 117 (12.26%) SIV, and 469 (49.16%) MET patients. In the study population, 83.65% were treated with endocrine therapy, starting an average of 179 days after index date. A total of 80.29% were treated with chemotherapy, starting an average of 53 days following the index date; 65% of patients initiated chemotherapy after index date without prior endocrine treatment. Rates varied by cohort: 90% of SIII, 57% of SIV, and 48% of MET patients had no evidence of endocrine treatment prior to initiating chemotherapy. Conclusions: This study found large proportions of HR+/HER2- advanced breast cancer patients initiated chemotherapy without prior endocrine therapy. Further investigation of patient characteristics and outcomes by therapy sequencing patterns will help illuminate the extent to which patterns adhere to NCCN guidelines.