Background: Durvalumab is an FDA-approved immunotherapy for the treatment of adults with UnResectable stage III non-small cell lung cancer (UR-NSCLC) without disease progression following concurrent chemoradiotherapy (CRT). There are limited real-world data regarding Durvalumab treatment initiation delays (TIDs) and reasons for them in the UR-NSCLC population. Methods: Patients with stage III UR-NSCLC receiving consolidation Durvalumab at the Veterans Health Administration (VHA) between January 1, 2017 and June 30, 2020 were selected from the VHA database using ICD-10, HCPCS, and J codes. All had the opportunity to be treated for 12 months and were followed from Durvalumab initiation through the earliest of their last VHA visit, loss to follow up, death, or the study’s end (and excluded if Durvalumab therapy was ongoing at the study’s end). Trained data abstractors determined the occurrence and reasons for TIDs (> 6 weeks from end of CRT to initiation of Durvalumab as in the PACIFIC trial) by chart review. Results: 935 patients were eligible for analysis (median age = 69 years; 95% males; 16% with ECOG performance status >1). TIDs occurred in 39% of the patients (Table). Durvalumab was initiated 61 days (median) from the end of CRT in TID patients vs. 31 days for those without TIDs. There were no significant (α<0.05) differences in age, race, smoking status, histology, or ECOG performance status and no comorbidity differences (except in patients with a history of cerebrovascular accident, for whom TIDs were more likely) between the TID/No-TID patients. Patients without timely post-CRT scans were more likely to have a TID. Of the 367 patients who experienced TIDs, 200 had documented reasons for the delay, consisting of other (not categorized) (28.5%), physician preference (20%), toxicity (11%), patient preference (10.5%), decline in performance status (10%), system issues (9.5%), social reasons (9%), and progression (0.5%). Conclusions: This is one of the largest retrospective cohort studies reporting real-world data in patients with UR-NSCLC receiving Durvalumab. TIDs were associated with increased time to post-CRT scans. This potential issue can be improved with care coordination and involvement of cancer navigators. Additional studies are needed to assess the impact of TIDs on survival outcomes.