Background: Chemotherapy has been a part of the evolving landscape of treatment for women with early-stage breast cancer (EBC). Over the last 20 years, standard treatment has included over 20 different drug combinations recommended by the National Comprehensive Cancer Network (NCCN), with nearly 45 variations on dosing and schedule across drug combinations. Little is known about the real-world landscape of chemotherapy used in EBC and how treatment patterns have changed with the advent of new standards of care. We evaluated chemotherapy patterns for women with EBC in the Optimal Breast Cancer Dosing (OBCD) Study, which uses electronic health record (EHR) data from KPNC (Kaiser Permanente Northern California) and Kaiser Permanente Washington (KPWA). Methods: In a cohort of 34,322 women diagnosed and treated for primary stage I-IIIA breast cancer aged 18+ at KPNC and KPWA, we explored patterns of chemotherapy use over time, from 2006 to 2019. We conducted stratified analyses based on receipt of pertuzumab and/or trastuzumab, due to their longer treatment duration. Results: Overall, 13,383 (39.0%) women received intravenous chemotherapy, with a decline observed in receipt of chemotherapy over time (40.5% in 2006 vs 35.9% in 2019) (p-trend<0.001). Of the women who received chemotherapy, 8.5% received neoadjuvant treatment, increasing from 4.1% in 2006 to 14.5% in 2019 (p-trend<0.0001). The average duration of chemotherapy was 5.4 months in 2006, increasing to 6.3 months in 2019 (p-trend<0.0001), and the number of chemotherapy infusion visits increased from an average of 10.7 visits in 2006 to an average of 12.5 in 2019 (p-trend<0.0001). Of those who received chemotherapy, 26.8% received trastuzumab or pertuzumab, with receipt of trastuzumab/pertuzumab increasing over time (p-trend<0.0001). For those receiving trastuzumab/pertuzumab, there was a decrease in treatment duration and average number of chemotherapy administration visits (p-trend<0.0001 for both). Conversely, for those not receiving trastuzumab/pertuzumab, there was an increase in the treatment duration and average number of visits (p-trend<0.0001 for both). Conclusions: While the prevalence of women receiving chemotherapy in our cohort has decreased since 2006, there has been a marked increase in neoadjuvant chemotherapy among those women receiving chemotherapy, as well as an increase in both the duration of chemotherapy treatment and number of associated chemotherapy administration visits, a pattern specifically observed in non-trastuzumab/pertuzumab containing regimens.The change in average duration and number of visits represents a shift in treatment patterns for patients and providers. Understanding trends in breast cancer chemotherapy is useful to inform clinical care and administrative planning for sites delivering chemotherapy for early-stage breast cancer.