Background: Studies have shown that SUDs not only impact cancer risk, treatment, and survivorship but also increase ED and hospital use. We sought to characterize SUD within Medicaid enrollees diagnosed with cancer in Washington state, hypothesizing thatMedicaid enrollees were more likely to be diagnosed with SUDs than commercial enrollees, and that patients with an SUD experienced higher rates of emergency department (ED) visits or inpatient (IP) stays during initial cancer treatment. Methods: We linked Washington State cancer registry records with claims data from two large commercial insurers and Medicaid. Eligibility criteria included patients under 65 diagnosed with a solid tumor between 2013-2021 who started systemic therapy within 6 months of diagnosis. SUD diagnoses were identified using codes from CMS’s hierarchical condition category for SUD in the 6 months following diagnosis. Chi-square tests were used to compare demographic characteristics of those with and without a SUD diagnosis. Multivariate logistic regressions were used to evaluate the association between SUD and likelihood of at least one ED visit or IP stay, after controlling for payer, sex, race, ethnicity, age, marital status, stage, cancer group, area deprivation index, and diagnosis year. Results: Among the 10,709 patients (6,680 Commercial, 3,718 Medicaid, 311 Multiple) who received systemic therapy, 7.2% had a SUD diagnosis. SUD patients were less likely to be Asian and more likely to be male, living without a partner, diagnosed at a later cancer stage, and living in more deprived areas. Medicaid patients had significantly higher rates of SUD than commercial-insured patients (14.3% vs 3.3%). Overall, 46.6% of cancer patients had an ED visit or IP stay. Patients with SUD had greater use of ED or IP visits than patients without SUD (70.1% and 44.8%, respectively). In the first 6 months of systemic therapy patients with SUD were significantly more likely to visit the ED or be admitted to IP (OR 2.12, 95% CI 1.79 - 2.50) than cancer patients without SUD. Conclusions: More than 1 in 7 Medicaid cancer patients in WA state has an identified SUD. SUD is associated with significant excess risk for ED/IP admissions during treatment. Accurate identification of SUD in patients is necessary prior to strategies aimed at mitigation of unplanned ED visits and hospitalizations during cancer care. Further research is needed on interventions that effectively reduce the risk during treatment.