BACKGROUND: Influx of large numbers of neutrophils is characteristic of pleural infection; however, neutrophil behaviour is understudied. We designed an observational multicohort study with the aims to discover and validate the association between neutrophil extracellular traps (NETs), disease severity, one-year survival, and the extent of sonographic septations. METHODS: We analysed five independent datasets across three countries. The PILOT cohort (UK, n=215) was used as a discovery cohort for the association of NETs with disease severity with validation in four independent cohorts across three countries (UK, Greece, Italy, n=100). The PILOT cohort was further analysed to assess the association of NETs with one-year survival and the development and extent of sonographic septations. A separate cohort (the Oxford-Osler cohort, UK, n=30) was used to assess the relationship between NETs and the need for intrapleural enzyme therapy (IET) as rescue treatment. RESULTS: A ten-unit increase in NETs was associated with 13% higher odds of being into a higher RAPID Score group (OR=1.13, 95% CI: 1.02-1.27, p=0.02), and this association remained robust in the independent validation cohort (OR=1.29, 95% CI: 1.11-1.40, p=0.0005). A ten-unit increase in NETs was also associated with a 15% higher risk of one-year mortality (HR=1.15, 95% CI: 1.01-1.32, p=0.04) independent of the RAPID Score, and a 17% higher likelihood of sonographic septations (OR=1.17, 95% CI: 1.01-1.37, p=0.04). Citrullinated fibrin was associated with septation severity (OR=1.10, 95% CI: 1.04-1.16, p=0.00005). CONCLUSIONS: Pleural fluid NETs are a biomarker of disease severity and one-year mortality, in pleural infection. Future studies are required to assess the combination of NET-targeting strategies in combination with existing treatments.