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Background: The occurrence and mechanisms of nocturnal hypoxemia in precapillary pulmonary hypertension (PH) are not clearly defined., Methods: In an observational, prospective, and transversal design, we studied 46 clinically stable patients with PH and a BMI < 35 kg/m(2), an FEV(1) > 60% predicted, and idiopathic pulmonary arterial hypertension (n = 29) or chronic thromboembolic pulmonary hypertension (n = 17). They underwent nocturnal polysomnography with transcutaneous capnography., Results: Most patients (69.6%) had New York Heart Association functional class II disease. Mean pulmonary artery pressure was 44 ± 13 mm Hg, and the cardiac index was 3.2 ± 0.6 L/min/m(2). Duration of sleep time spent with oxygen saturation as measured by pulse oximetry <90% was 48.9% ± 35.9%, and 38 of 46 patients (82.6%) had nocturnal hypoxemia. Mean apnea-hypopnea index was 24.9 ± 22.1/h, and 41 patients (89%) had sleep apnea. The major mechanism of nocturnal hypoxemia was a ventilation/perfusion mismatch alone or associated with obstructive apneic events. Multivariate logistic regression identified both FEV(25%-75%) (OR, 0.9519; 95% CI, 0.9089-0.9968; P = .036) and mean pulmonary artery pressure (OR, 1.1068; 95% CI, 1.0062-1.2175; P = .037) as significant predictors of nocturnal hypoxemia. Clinical symptoms were not predictive of nocturnal hypoxemia., Conclusions: The occurrence of nocturnal hypoxemia is high in PH and should be screened for systematically. Further studies are needed to determine the impact of nocturnal hypoxemia on the outcome of patients with PH., Trial Registry: ClinicalTrials.gov; No.: NCT01371669; URL: www.clinicaltrials.gov