Background: Chronic mountain sickness (CMS) is characterized by a loss of adaptation to hypoxia in high-altitude (HA) dwellers. Chronic hypoxemia, excessive erythrocytosis and frequently pulmonary hypertension (PH), which may lead to cardiac failure, develop in patients. We sought to assess the determinants of cardiac function in CMS patients with hypoxia-induced PH. Methods: Fifteen healthy men living at sea level (SL) were compared to 15 healthy men living at HA and 55 patients with CMS from Cerro de Pasco, Peru (altitude, 4,300 m). Pulmonary pressures and cardiac function were estimated by echocardiography. Results: None of the subjects had overt cardiac failure symptoms. CMS patients exhibited elevated mean pulmonary pressures as assessed by high-tricuspid pressure gradients (CMS patients, 34 ± 10 mm Hg; HA subjects, 25 ± 4mmHg [p = 0.002]; and SL subjects, 19 ± 3mmHg [p < 0.001]). They also showed right ventricular (RV) dilation (mean end-diastolic RV area: CMS patients, 17 ± 2 cm2; HA subjects, 13 ± 2cm2; SL subjects, 12 ± 2cm2;p< 0.001) but did not display impaired systolic ventricular function. However, the RV Tei index was increased in CMS and HA subjects (CMS patients, 0.56 ± 0.15; HA subjects, 0.52 ± 0.12; SL subjects, 0.21 ± 0.12; p < 0.001). Conclusion: Despite obvious pulmonary arterial hypertension and right heart dilation, CMS patients did not show any symptom or echocardiographic parameter of heart failure. copyright © 2009 American College of Chest Physician.