Introduction/Objective. In the present paper, we aimed to test the value of the red cell distribution width (RDW) coefficient of variation as a candidate biomarker for pulmonary arterial hypertension (PAH) in patients with connective tissue disorders (CTD), correlating it with the degree of cardiopulmonary impairment in these patients. Methods. The study population included N=141 patients with CTD and N=59 patients affected by pulmonary hypertension of other etiologies, all referred to the Pulmonary Hypertension Clinic of the Cardiology Division of an Academic Hospital in Northern Italy for evaluation (including right catheterization). Clinical, instrumental, and laboratory data were collected and related to RDW and other full blood count indexes. Results. Twenty out of 141 CTD patients (14%) received a diagnosis of PAH. In comparison to those without PAH, CTD patients with PAH displayed a larger RDW (14.9% (13.5-17.2) vs. 13.8% (13.1-15.0); p=0.02) and a lower platelet count (205 177‐240×109/l vs. 244 197.5‐304.2×109/l; p=0.005). Moreover, with respect to CTD patients without PAH, RDW was significantly larger also in PH of other etiologies. In contrast, the platelet count was significantly lower only in CTD-related PAH, with a value>276×109/l being 100% sensitive in ruling out PAH. Finally, RDW, but not the platelet count, was related directly to systolic pulmonary arterial pressure (r=0.381; p=0.0008) and right ventricle diameter (r=0.283; p=0.015) and inversely to diffusing capacity of the lung for carbon monoxide (r=−0.325; p=0.014). Conclusion. RDW is a promising candidate biomarker for the screening and the prognostic stratification of PAH in CTD patients.