Background: The neutrophil to lymphocyte ratio (NLR) and platelet to lymphocyte ratio (PLR) has been of prognostic interest in lung cancer. Sustained NLR and PLR after initial chemoradiotherapy (CRT) has been shown to correlate with worse prognosis in other solid organ malignancies. This study aims to add to the available evidence by describing the survival outcomes for patients with stage III NSCLC who are treated with consolidation Durvalumab when stratified by baseline or sustained NLR and PLR. Methods: We conducted a retrospective observational cohort study involving 6 sites across Sydney, Australia, including all patients diagnosed with stage III NSCLC treated with chemoradiation (CRT) and at least one cycle of durvalumab between January 2018 to September 2021. Patients had NLR and PLR collected prior to their first treatment of CRT and prior to their first treatment with Durvalumab. We used NLR and PLR values of 3 and 185 respectively to stratify patients into high and low groups. Patients with sustained NLR or PLR were defined as those with values > = 3 or > = 185 at both pre-CRT and pre-Durvalumab time points. Results: 148 patients were included in the study. The median follow-up from the start of Durvalumab was 15.1 months. The median age was 66 years. 61% (n = 90) of patients were male. The median PFS was 17.6 months in the pre-CRT NLR high cohort and not reached in the pre-CRT NLR low cohort (HR 1.99; 95% CI 1.16 – 3.41; p = 0.01). Median OS was 35.5 months versus 42.0 months in high and low pre-CRT NLR groups respectively (HR 2.62; 95% CI 1.23 – 5.56; p < 0.01). The median PFS was 19.9 months in the pre-CRT high PLR cohort versus and not reached in the pre-CRT low PLR cohort (HR 1.98; 95% CI 1.15 – 3.42; p = 0.02). The median OS was 39.9 months versus 42.0 months in high and low pre-CRT PLR groups respectively (HR 2.29; 95% CI 1.08– 4.88; p = 0.03). Median PFS for sustained NLR elevation was 17.1 months versus NR (HR 1.5, 95% CI 1.1 – 2.2, p < 0.01). Similarly median PFS for sustained PLR elevation was 16.6 months versus NR (HR 1.7, 95% CI 1.1 – 2.4, p < 0.01). Conclusions: Pre-CRT NLR and pre-CRT PLR are associated with OS and PFS outcomes in stage III unresectable NSCLC treated with CRT and Durvalumab. Those with sustained NLR or sustained PLR also have worse progression free survival outcomes.