Background: Emerging data indicates that extracellular traps (ETs), structures formed by various immune cell types, may contribute to the pathology of noninfectious infammatory diseases. Histone hypercitrullination is an important step in ETs formation and citrullinated histone H3 (H3cit) is considered a novel and specifc biomarker of that process. In the present study we have evaluated circulating H3cit in stable asthmatics and investigated its relationship with asthma severity, pulmonary function and selected blood and bronchoalveolar lavage (BAL) biomarkers. Methods: In 60 white adult stable asthmatics and 50 well-matched controls we measured serum levels of H3cit. In asthmatics we also performed bronchoscopy with BAL. We analyzed blood and BAL biomarkers, including interleukin (IL)-4, IL-5, IL-6, IL-10, IL-12p70, IL-17A and interferon γ. For statistical analysis, Mann-Whitney U-test, χ2 test, one-way ANCOVA, ROC curve analysis and univariate linear regression were applied. Independent determinants of H3cit were established in a multiple linear regression model. Results: Asthma was characterized by elevated circulating H3cit (17.49 [11.25–22.58] vs. 13.66 [8.66–18.87] ng/ml, p=0.03). In asthmatics positive associations were demonstrated between serum H3cit and lung function variables, including total lung capacity (TLC) (β=0.37 [95% CI 0.24-0.50]) and residual volume (β=0.38 [95% CI 0.25–0.51]). H3cit was increased in asthma patients receiving systemic steroids (p=0.02), as well as in subjects with BAL eosino‑ philia above 144 cells/ml (p=0.02). In asthmatics, but not in controls, circulating H3cit correlated well with number of neutrophils (β=0.31 [95% CI 0.19–0.44]) and monocytes (β=0.42 [95% CI 0.29–0.55]) in peripheral blood. Further‑ more, BAL macrophages, BAL neutrophils, TLC, high-sensitivity C-reactive protein, Il-12p70 and bronchial obstruction degree were independent determinants of H3cit in a multivariate linear regression model. Conclusions: Asthma is characterized by increased circulating H3cit likely related to the enhanced lung ETs forma‑ tion. Inhibition of ETs might be a therapeutic option in selected asthma phenotypes, such as neutrophilic asthma.