Endothelin-1 (ET-1) is overexpressed in pulmonary arteries of pulmonary hypertension (PH) patients and contributes to the sustained vasoconstriction, remodeling process, and thrombosis of vessels that underlie development and progression of this disease. Increased circulating ET-1 correlates with markers of PH severity, and ET-1 is regarded as a potential diagnostic and prognostic biomarker in PH. Because the within-individual variability measured in PH patients and in healthy subjects contributes to determine the biomarker predictive value and must be taken into account to establish cutoff values, we determined the short-term variability of circulating ET-1 in controls and in PH patients.