Background: Oral endocrine therapy (ET) is the cornerstone of adjuvant therapy for hormone receptor-positive (HR+) breast cancer, greatly reducing the risk of recurrence and death when taken for at least 5 years. Failure to initiate ET is a known problem that adversely affects recurrence and survival. The survival gap between black and white women with breast cancer is most pronounced in HR+ subtypes, and could be related to differences in ET use. The relationship between race and failure to initiate ET is not well defined. We investigated patterns of ET initiation by race in a diverse cohort of women covered by commercial health insurance. Methods: We identified 2753 women with incident HR+ breast cancer in the North Carolina Central Cancer Registry whose records linked to insurance claims using the statewide Integrated Cancer Information and Surveillance System (ICISS) database maintained by the University of North Carolina. The sample included privately insured women age <65 years who were diagnosed with stage I-III HR+ breast cancer between 2004 and 2009 and received primary therapy including breast surgery within 12 months of diagnosis. We used multivariate Poisson regression with robust variance to examine the effect of race on likelihood of initiating ET (tamoxifen, anastrozole, letrozole or exemestane). Results: 14% of eligible women did not initiate any form of ET within 12 months of diagnosis. Black women initiated ET at a significantly lower rate (IRR 0.83, 95% CI 0.73-0.93) after adjustment for age, disease characteristics, other treatments, and socioeconomic variables. Women receiving breast conserving surgery (BCS) without radiation (RT) were less likely to initiate compared to those receiving BCS + RT or mastectomy, and women whose treatment included chemotherapy were less likely to initiate ET. Conclusions: Black women in our sample were significantly less likely to initiate ET in a timely fashion than their white counterparts. This underutilization may contribute to the pronounced survival gap among black women with HR+ breast cancer, and represents an opportunity for intervention to reduce breast cancer disparities. Further investigation is needed to understand the reasons why black women do not initiate ET.