ObjectiveWe aimed to utilize a nationally representative database to study the effect of Medicaid expansion on the receipt of adjuvant chemotherapy in eligible patients. Materials and MethodsRetrospective review of the National Cancer Database (NCDB) was performed between 2006 and 2019. Patients with clinical T1-T3, N1, and M0 were included. Patients with nodal disease or tumors >4 cm were eligible for adjuvant therapy. Demographic and clinical information were collected. A difference-in-difference analysis was performed to compare changes in the rate of adjuvant chemotherapy. Results9,954 eligible patients were treated in states that expanded Medicaid coverage in January 2014 or later, with 4,809 patients treated in the pre-expansion years (2012-2013) and 5,145 patients treated in the post-expansion years (2017-2018). Following Medicaid expansion, eligible patients were more likely to receive adjuvant therapy (70.2% vs. 62.3%; p<0.001). Compared with the pre-expansion period, patients who received adjuvant therapy were more likely to use Medicaid insurance post-expansion (7.8% vs. 5%, p<0.001). Among patients using Medicaid coverage only, a greater percentage started adjuvant therapy within 8 weeks of resection following Medicaid expansion (46.6% vs. 38.3%, p=0.048). The observed difference-in-difference in the change in adjuvant therapy rate from the pre-expansion period to the post-expansion period between expansion and non-expansion states was 1.25% (95% Bootstrap CI -0.36%–3.18%). There was a modest survival benefit in expansion states post-expansion. ConclusionMedicaid expansion appears to be associated with increased access to care, as shown by the increased receipt of adjuvant systemic therapy in eligible patients. Micro-AbstractA nationally representative sample was used to study the effect of Medicaid expansion on the receipt of adjuvant chemotherapy. There was an increase in the receipt of adjuvant chemotherapy from the pre-expansion to the post-expansion period between expansion and non-expansion states. Medicaid expansion seems to be associated with increased access to care, as shown by the increased use of chemotherapy.