Objectives: Aromatase inhibitors (AIs) are standard adjuvant therapy for postmenopausal women with early-stage, estrogen receptor–positive breast cancer. We designed our study to determine whether women initiating adjuvant therapy with an AI underwent baseline bone mineral density testing, as well as what factors predicted adherence with testing guidelines. Methods: Medicare Parts A, B, and D claims were used to identify a cohort of women aged 67 years and older with incident breast cancer in 2006 and 2007 who started AI therapy. Medicare claims provided information about bone density testing, as well as demographic and other treatment data through 2012. We also ascertained which patients were treated with bisphosphonates and studied the relationship of bisphosphonate therapy with bone density testing. Results: Approximately two-thirds of patients had baseline bone density testing. Older age, comorbidity, low income, and black race were associated with lower rates of baseline bone density testing. Testing rates decreased substantially with increasing age from 73% for women aged 67 to 70 years to 51% for those 85 years of age and older (adjusted odds ratio for not being tested, 2.48 [Cl, 2.17–2.82]). The proportion of women who had neither bone density testing nor bisphosphonate therapy increased with age as well. Conclusions: Despite the importance of age as a risk factor for fractures, older women starting treatment with AIs for treatment of breast cancer are less likely to undergo recommended bone density assessment.