BACKGROUND: New oral oncology medications bring novel challenges when patients are initiating treatment. Rates of primary medication nonadherence (PMN), the rate at which a medication is prescribed but not obtained, of up to 30% have been reported for oral oncology medications. More research is needed to identify causes and develop strategies for health system specialty pharmacies (HSSPs) to improve cancer treatment initiation rates. OBJECTIVE: To evaluate the rate and reasons for PMN to specialty oral oncology medications in an HSSP setting. METHODS: We performed a multisite retrospective cohort study across 7 HSSP sites. Patients were included if they had an orally self-administered oncology medication referral generated by the health system of the affiliated specialty pharmacy between May 1, 2020, and July 31, 2020. Data collected at each site using pharmacy software and the electronic health record were deidentified and aggregated for analysis. After identifying unfilled referrals within a 60-day fill window, a retrospective chart review was performed to identify final referral outcomes and reasons for unfilled referrals. Referral outcomes were categorized as unknown fill outcomes (because of being referred to another fulfillment method or if received for benefits investigation only), filled by the HSSP, or not filled. The primary outcome was PMN for each PMN-eligible referral and secondary outcomes included reason for PMN and time to fill. The final PMN rate was calculated by dividing the number of unfilled referrals by total referrals with a known fill outcome. RESULTS: Of 3,891 referrals, 947 were PMN eligible, representing patients with a median age of 65 years (interquartile range = 55-73), near equal distribution between male and female (53% vs 47%), and most commonly with Medicare pharmacy coverage (48%). The most referred medication was capecitabine (14%), and the most common diagnosis was prostate cancer (14%). Among PMN-eligible referrals, 346 (37%) had an unknown fill outcome. Of the 601 referrals with known fill outcome, 69 referrals were true instances of PMN, yielding the final PMN rate of 11%. Most referrals were filled by the HSSP (56%). Patient decision was the most common reason for not filling (25%; 17/69 PMN cases). The median time to fill after initial referral was 5 days (interquartile range = 2-10). CONCLUSIONS: HSSPs have a high percentage of patient initiation of new oral oncology medication treatments in a timely manner. More research is needed to understand patient reasons for deciding not to start therapy and to improve patient-centered cancer treatment planning decisions. DISCLOSURES: Dr Crumb was a planning committee member with Horizon CME for the Nashville APPOS 2022 Conference. Dr Patel received funding and support for attending meetings and/or travel from the University of Illinois Chicago College of Pharmacy.