Background The neutrophil-to-lymphocyte ratio (NLR) as a marker of systemic inflammation has been associated with worse prognosis in several chronic disease states, including heart failure. However, few data exist on the prognostic impact of elevated baseline NLR or change in NLR levels during follow-up in patients undergoing transcatheter or surgical aortic valve replacement (TAVR or SAVR) for aortic stenosis. Methods and Results NLR was available in 5881 patients with severe aortic stenosis receiving TAVR or SAVR in PARTNER (Placement of Aortic Transcatheter Valves) I, II, and S3 trials/registries (median [Q1, Q3] NLR, 3.30 [2.40, 4.90]); mean NLR, 4.10; range, 0.5-24.9) and was evaluated as continuous variable and categorical tertiles (low: NLR ≤2.70, n=1963; intermediate: NLR 2.70-4.20, n=1958; high: NLR ≥4.20, n=1960). No patients had known baseline infection. High baseline NLR was associated with increased risk of death or rehospitalization at 3 years (58.4% versus 41.0%; adjusted hazard ratio [aHR], 1.39; 95% CI, 1.18-1.63; P<0.0001) compared with those with low NLR, irrespective of treatment modality. In both patients treated with TAVR and patients treated with SAVR, NLR decreased between baseline and 2 years. A 1-unit observed decrease in NLR between baseline and 1 year was associated with lower risk of death or rehospitalization between 1 year and 3 years (aHR, 0.86; 95% CI, 0.82-0.89; P<0.0001). Conclusions Elevated baseline NLR was independently associated with increased subsequent mortality and rehospitalization after TAVR or SAVR. The observed decrease in NLR after TAVR or SAVR was associated with improved outcomes. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT00530894, NCT0134313, NCT02184442, NCT03225001, NCT0322141.