1. 1. Thirty-two healthy children aged 1 to 14 years, born and living at high altitudes were studied by means of right heart catheterization. They lived in Morococha and Cerro de Pasco, the former at an altitude of 14,900 feet, the latter at 14,200. 2. 2. Mild pulmonary hypertension and an increased pulmonary vascular resistance have been found in children born and living in high altitudes. Values of cardiac output and pulmonary wedge pressure were similar to those reported at sea level. The increased pulmonary vascular resistance is ascribed to structural changes reported by other investigators in the small pulmonary arteries and arterioles. 3. 3. Pulmonary hypertension is greater in children than in adults of high altitudes, and it is greater in children under 5 years of age than in those 6 to 14. In the younger group pulmonary pressures are similar to those obtained in newborns at sea level. This signifies that at high altitudes there is a delay in the evolution of the pulmonary pressures with aging, in contrast to what occurs at sea level. This slow evolution agrees with the slow regression of right ventricular hypertrophy at high altitudes as demonstrated by anatomic, electrocardiographic and vectorcardiographic studies. 4. 4. The pulmonary hypertension found at high altitudes could be considered to a certain extent as a form of primary pulmonary hypertension; however, basic differences are pointed out between both conditions. The role of pulmonary hypertension in the high incidence of patent ductus arteriosus at high altitudes is discussed. © 1963.