Ten male subjects with chronic mountain sickness were studied in Cerro de Pasco, Perú at 14,200 feet above sea level. Cyanosis, extreme polycythemia and very low values of arterial oxygen saturation were frequent findings. Hypoxia and polycythemia of severe degree are related to alveolar hypoventilation demonstrated in previous studies. Roentgen examination as well as electrocardiographic and vectorcardiographic studies showed enlargement of the right cardiac chambers. Pulmonary hypertension, of greater degree than seen in healthy highlanders, was found in these patients. Muscularization of pulmonary arterioles, hypoxic arteriolar vasoconstriction and polycythemia are contributing factors to the mechanism of pulmonary hypertension. The importance of the functional factors is demonstrated by the prompt disappearance of clinical symptoms and the great reduction of right cardiac overload and pulmonary hypertension in the patients moved down to sea level. The clinical symptoms as well as the roentgenologic, electrocardiographic and hemodynamic data are similar to those occurring in cases of chronic cor pulmonale due to alveolar hypoventilation. Muscularization of the pulmonary arteries and reversion of clinical and physiologic findings are also features common to the hypoxic type of chronic cor pulmonale. There is therefore enough clinical, physiologic and anatomic basis to conclude that Monge's disease is a variety of chronic cor pulmonale due to alveolar hypoxia. © 1971.