OBJECTIVES: Although adjuvant endocrine therapy (AET) is becoming an important cornerstone of treatment in Medicare breast cancer survivors, few studies have examined variations in the use of this treatment by potentially important sociodemographic influences like patient race and geography are independent of economic drivers such as patient costs associated with these medications. METHODS: To test our hypotheses that utilization may be driven by sociodemographics independent of drug costs, we compared and contrasted AET therapy use in two survivor populations with breast cancer, those enrolled in Medicare Part D (with significant copayment and medication cost issues) with survivor populations enrolled in the Medicaid program (with little to no copayments for these medications). We compared 18,075 Medicare breast cancer survivors enrolled in Part D plans with 1,307 Medicaid survivors in three states (West Virginia, Maryland and Ohio) over a 5 year period. RESULTS: Similar to what we found in the SEER-Medicare data, white patients had significantly higher use rate of AET than African American patients (6.6% vs. 4.6%, p<0.0001), while other minorities had significantly higher use rate of AET than white patients (8.5% vs. 6.6%, p = 0.02). These were consistent with what we found in the SEER-Medicare data. We also identified significant differences in the use rate of AET among the three states, which have different geographies (use rate of AET in WV, MD, and OH were 10.8%, 7.0%, and 4.7%, respectively (p<0.0001). These findings were confirmed in multivariate models. CONCLUSIONS: Our analyses of data from two different insurance programs with differential patient economic cost burden for AET therapy finds evidence that there are distinct sociodemographic factors such as patient race and rural status that affect the use of AET therapy, and these effects are independent of patient cost-sharing for these medication therapies.