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, 1-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 and Italy, n=100). The PILOT cohort was further analysed to assess the association of NETs with 1-year survival and the development and extent of sonographic septations. A separate cohort (Oxford-Osler, UK, n=30) was used to assess the relationship between NETs and the need for intrapleural enzyme therapy as rescue treatment. Results A 10-unit increase in NETs was associated with 13% higher odds of being in 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 10-unit increase in NETs was also associated with a 15% higher risk of 1-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 1-year mortality, in pleural infection. Future studies are required to assess the combination of NET-targeting strategies in combination with existing treatments.