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Physiologic and anatomic changes have been demonstrated in healthy children born and living at high altitudes. Electrocardiographic and vectorcardiographic studies indicate that after birth the right ventricular preponderance remains throughout infancy and childhood. Anatomic observations have confirmed the presence of right ventricular hypertrophy in healthy children of high altitudes. Cardiac catheterizations reveals mild pulmonary hypertension associated with increased pulmonary vascular resistance, the degree of pulmonary hypertension being higher under 5 years of age. The increased pulmonary resistance is principally related to structural changes of the pulmonary vasculature, while arteriolar vasoconstriction and polycythemia are secondary factors. Pulmonary hypertension and right ventricular hypertrophy are not associated with any kind of symptoms in high-altitude children, and we must understand these changes as related in some way to the mechanisms of natural acclimatization. Patent ductus arteriosus is more frequent at high altitudes than at sea level, and the incidence augments gradually as altitude above sea level increases, the relationship following a curve of parabolic type. It is probable that at high altitude hypoxia and pulmonary hypertension, which remain as chronic conditions after birth, are factors related to the patency of ductus arteriosus. High altitude pulmonary edema may occur in healthy children returning to altitude after being at sea level for a few days or weeks. Rapid improvement of the clinical condition is obtained after continuous oxygen administration; however, death may occur if early treatment is not given. Cardiac catheterization reveals a normal pulmonary wedge pressure and a degree of pulmonary hypertension two or three times greater than that corresponding to healthy children of high altitude. The mechanism of high altitude pulmonary edema is discussed.