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Serum chloride is an important homeostatic biomarker in left heart failure, with significant prognostic implications. The impact of serum chloride in the long-term survival of patients with pulmonary arterial hypertension (PAH) is unknown. We tested whether serum chloride levels are associated with long-term survival in patients with PAH.We included patients with idiopathic or heritable PAH who had a basic metabolic panel performed at the time of their diagnostic right heart catheterization. Laboratory results were recorded both at diagnosis and 6-month follow-up.We included 277 patients, mean age 51 ± 18 years and 73% women, of whom 254 had a follow-up electrolyte determination at 6 months. Serum chloride was 102.9 ± 3.9 mM/L at diagnosis. A serum chloride ≤ 100 mM/L was noted in 65 (24%) and 53 (21%) patients at diagnosis and 6 months, respectively. Patients with serum chloride ≤ 100 mM/L at 6 months tracked with increase mortality when adjusted by age, sex, pulmonary vascular resistance, diuretics or prostacyclin analogs usage, and serum creatinine and sodium at 6 months (hazard ratio, 1.83; 95% CI, 1.11-3.00). This group of patients was older, with decreased functional capacity, had worse renal function, took more diuretics, had higher pulmonary artery wedge pressure but lower mean pulmonary artery pressure, transpulmonary gradient, and pulmonary vascular resistance.Serum chloride at 6 months from the PAH diagnosis is a strong and independent predictor of mortality in patients with idiopathic or heritable PAH, even after adjusting serum sodium, renal function, diuretic, and prostacyclin analog usage.