Introduction: Platelets are known to contribute to inflammatory processes alongside their role in thrombosis. There is evidence to suggest that inflammation and thrombocytosis are linked to poor outcomes in COPD. This study investigated the role of platelets in bronchiectasis.Methods: Prospective observational study of patients with HRCT confirmed bronchiectasis (2012-2015). Severity of disease was assessed by the bronchiectasis severity index(BSI). Thrombocytosis was defined as baseline platelet count >400 x 109 cells/mm3. Outcomes were mortality, hospital admissions, exacerbations, quality of life and lung function. Platelets were identified by flow cytometry (positive staining for CD41) and soluble factors assayed by ELISA in spontaneous sputum.Results: 287 patients were included. Thrombocytosis was identified in 12.9%. These patients had more frequent exacerbations (3.0 standard deviation-sd 2.1 vs 1.9 sd 1.7, p=0.001), worse quality of life using the SGRQ (56.1 sd 22.1 vs 42.7 sd 21.2) and a higher bronchiectasis severity index (9.9 sd 6.0 vs 7.2,sd 4.5, p=0.001)Even after adjustment for confounders, these patients had significantly higher mortality (adjusted hazard ratio 2.9 95% CI 1.1-7.8).Platelet-neutrophil aggregates were identified in a subset bronchiectasis sputum, and markers of platelet activation (serotonin and thromboxanes) could be identified at low concentrations in bronchiectasis sputum samples.Conclusion: Thrombocytosis is associated with disease severity in bronchiectasis patients. The data suggest a possible link between systemic inflammation and airway neutrophil activation.