e18055 Background: Systematic inflammation plays an integral role in tumor development and metastasis. High neutrophil counts, among various peripheral blood biomarkers, have been shown to be associated with worse survival outcomes in multiple cancer types. To evaluate the absolute neutrophil count (ANC) as a prognostic biomarker in head and neck cancer, we performed an observational cohort study of patients undergoing definitive chemoradiation. Methods: Our institutional database at the Roswell Park Comprehensive Cancer Center was queried for patients with non-metastatic head and neck cancer who underwent definitive chemoradiation between June 2007 and April 2023. ANC was analyzed as a continuous variable. Survival outcomes were assessed using Kaplan-Meier method, log-rank tests, Cox proportional hazard multivariable (MVA) analyses. Fine-Gray MVA was used to analyze failure outcomes with death as a competing event. To reduce selection bias, propensity score matching was performed using ANC as a dichotomous variable with its median value as a cutoff. Subgroup analyses were performed among p16-negative and p16-positive cohorts. Results: A total of 668 patients who met our criteria. Median follow up was 30.3 months (interquartile range 9.9-60.7). Elevated ANC as a continuous variable was associated with worse overall survival (OS; adjusted hazards ratio [aHR] 1.02, 95% confidence interval [CI] 1.01-1.04, p=0.003), progression free survival (PFS; aHR 1.03, 95% CI 1.01-1.04, p<0.001), and distant failure (DF; aHR 1.03, 95% CI 1.01-1.05, p=0.007), but not locoregional failure (LRF; aHR 1.02, 95% CI 1.00-1.05, p=0.10). Similar findings were noted among 278 matched pairs (OS: hazards ratio [HR] 1.66, 95% CI 1.23-2.26, p=0.001; PFS: HR 1.74, 95% CI 1.31-2.29, p<0.001; LRF: HR 1.66, 95% CI 0.98-2.80, p=0.06; DF: HR 1.64, 95% CI 1.03-2.60, p=0.04). On subgroup analysis, among 118 patients with p16-negative tumors, elevated ANC as a continuous variable was not associated with OS (aHR 1.00, 95% CI 0.90-1.11, p=0.97) and PFS (aHR 0.97, 95% CI 0.87-1.08, p=0.54). The number of LRF (n=21) and DF (n=17) were too few to analyze in this subgroup. However, among 344 patients with p16-positive tumors, high ANC was associated with worse OS (aHR 1.03, 95% CI 1.00-1.06, p=0.046), PFS (aHR 1.05, 95% CI 1.02-1.07, p<0.001), LRF (aHR 1.08, 95% CI 1.05-1.12, p<0.001), and DF (aHR 1.04, 95% CI 1.01-1.08, p=0.02). Conclusions: Our study suggested that elevated ANC was an independent, adverse prognostic factor for worse survival and distant metastasis outcomes. Similar findings were also observed among patients with p16-positive tumors, with high ANC also associated with worse locoregional failure. Further studies are warranted to tailor interventions based on such biomarkers.