Background: Bisphenol A (BPA), a widely used chemical in plastics production, has been shown to have estrogenic activity, although it is unclear whether BPA exposure alters effectiveness of aromatase inhibitor (AI) treatment among women with breast cancer. In vitro studies have indicated that at higher concentrations (≥10^-4μM), BPA inhibits aromatase expression and activity, whereas at environmentally relevant concentrations (≤10^-8μM), BPA increases aromatase expression and activity. Few in vivo studies have investigated the association between BPA exposure and estrogen concentrations among non-pregnant women. This pilot study among post-menopausal women receiving AI therapy evaluated whether BPA is detectable in participants’ urine, and the association between BPA and hormone concentrations. Methods: This study was ancillary to a larger intervention study (ClinicalTrials.gov NCT01509079), and participants provided informed consent for this ancillary study under a protocol approved by the institutional review boards of Park Nicollet Institute/HealthPartners and the University of Minnesota. AI adherence during the 4 weeks prior to the baseline visit was monitored through weekly diaries. Participants were instructed to collect spot urine collections in sterile, BPA-free polypropylene containers in the 24 hours prior to the baseline study visit. Serum hormone concentrations were measured using serum collected at the baseline study visit. The Breast Cancer Prevention Trial - Musculoskeletal Symptom (BCPT-MS) scale was used to assess AI-associated musculoskeletal syndrome symptoms at the baseline study visit. Results: Fourteen women agreed to participate in the pilot study, however one participant’s data was excluded due to substances interfering with BPA measurement. BPA was detected in urine from all 13 remaining participants. Geometric mean urinary BPA concentration (1.55 mcg/g creatinine, 95% confidence interval (CI): 0.98-2.45) was higher than concentrations reported for females (1.36 mcg/g creatinine, 95% CI: 1.23-1.51) in the 2013-14 National Health and Nutrition Examination Survey biomonitoring data. Age, body mass index, and percent body fat did not differ between women above or below the study median urinary BPA concentrations (1.49 ng/mL). Women with BPA concentrations >1.49 ng/mL had consistently lower serum estrone (5.0 vs. 7.0 pg/mL, p=0.15), estradiol (2.8 vs. 3.6 pg/mL, p=0.04), and testosterone (21.5 vs 23.8 ng/dL, p=0.28) concentrations compared to women with lower BPA concentrations. No statistically significant differences in BCPT-MS scores were observed between women with higher vs. lower urinary BPA concentrations. Conclusions: These preliminary findings suggest that BPA exposures may work in concert with AIs to lower serum estrogen levels. However, further research is needed to confirm these findings.