41 Background: Oral anti-cancer drugs (OACDs) have become increasingly prescribed over the last 10 years and require a significant amount of care coordination. Preliminary administrative database studies have shown that 10-15% of prescriptions are never received by the patient, but the reasons behind this are poorly understood. In this study, we prospectively identified failure to receive (FR) cases in which OACD prescriptions were never received by patients, examined underlying reasons for FR, and assessed clinical and process-related factors associated with FR. Methods: We prospectively collected data on new OACD prescriptions for adult oncology patients at a large, urban academic cancer center from 1/1/2018 to 12/31/2019. We collected patient demographic, clinical, and insurance data, OACD delivery date, and interactions with payers and financial assistance groups. FR was defined as failure to receive a prescribed OACD. Reasons for FR were confirmed by manual chart review and classified into seven categories: clinical deterioration, financial access, provider-driven clinical decision making, patient-directed change, transfer of care, lost to follow up, and other. We calculated the relative proportion of each FR category and used multivariable logistic regression to identify factors associated with FR, including initiation of a prior authorization and drug class. Results: The cohort included 1,080 patients who were prescribed 1,269 new OACDs. Of these prescriptions, 13% (N=163) were categorized as FR. Among the 158 patients with FR, average patient age was 66 years, 55% identified as non-Hispanic white, 61% had any Medicare plan, 11% had Medicaid only, and 25% had commercial insurance. Overall, 18% of FR cases were attributed to clinical deterioration, 13% to financial access, 29% to provider-driven clinical decision making, 17% to patient-directed change, 13% to transfer of care, and 5% were lost to follow up. Univariate analysis showed that FR was less likely in cases where prior authorization was initiated (p < 0.001) and multivariate analysis confirmed this result (OR 0.47 [CI 0.33-0.66], p < 0.001). Conclusions: Though the majority of oncology patients prescribed OACDs received the drug, 13% of patients in our study experienced FR. FR is associated with a lack of prior authorization initiation, which may reflect barriers to access, a change in clinical decision-making, or patient choice. Ultimately, FR is multifactorial and may be appropriate in some cases. More work is needed to determine whether improved access would increase uptake in some patients. [Table: see text]