['Chronic thromboembolic pulmonary hypertension (CTEPH) is a life-threatening condition characterized by single or recurrent pulmonary embolisms, which promotes pulmonary vascular remodeling. There is significant relationship between red cell distribution width (RDW) and pulmonary hypertension; however, the usefulness of RDW as biomarker for the diagnosis of CTEPH remains poorly defined.', 'This study sought to assess the change and the diagnostic value of RDW in CTEPH.', 'This retrospective study included 56 CTEPH patients and 56 sex- and age-matched healthy controls treated at Beijing Chao-Yang Hospital. Correlations between RDW and hematological and hemodynamic parameters were assessed. A logistic regression model was applied to test independent parameters in relation to the diagnosis of CTEPH, and receiver operating characteristic curve was plotted to determine the diagnostic value of RDW in CTEPH.', 'RDW values were significantly higher in CTEPH patients (13.82%\u2009±\u20091.14%) compared with healthy controls (12.75%\u2009±\u20090.49%) and in World Health Organization (WHO), functional class III-IV cases (14.39%\u2009±\u20091.24%) compared with class I-II cases (13.32%\u2009±\u20090.78%) (both P\u2009=\u20090.000). RDW levels in CTEPH patients showed negative correlations with hemoglobin (r\u2009=\u2009-0.357, P\u2009=\u20090.007) and cardiac index (r\u2009=\u2009-0.288, P\u2009=\u20090.031), and positive correlations with pulmonary vascular resistance (r\u2009=\u20090.292, P\u2009=\u20090.029) and WHO functional class (r\u2009=\u20090.450, P\u2009=\u20090.001). Moreover, RDW was an independent parameter in the diagnosis of CTEPH (95% confidence interval: 2.866-13.698, P\u2009=\u20090.000); in particular, an RDW level\u2009≥\u200913.05% was the most useful cut-off value, with a sensitivity of 82.1% and a specificity of 71.4%.'] Is red cell distribution width increased in chronic thromboembolic pulmonary hypertension?