RADIOGRAPHIC evidence of right ven-tricular enlargement with bulging of the main pulmonary artery1-3 and electroveetoreardiographic signs of right ventricular preponderance2'4-6 arecommon findings in people born and living at high altitudes. Ana-tomically, the existence of right ventricular hypertrophy has been demonstrated both in adults7-9 and children. 10 Recently, hemodynamic studies have disclosed that these people present, at rest, a mild degree of pulmonary hypertension and increased pulmonary vascular resistance."-'3 Children below 5 years of age show higher pulmonary pressure values than older children and adults.'3 Furthermore, different from what occurs in normal persons at sea level, the pulmonary pressure increases markedly during exercise. 14 The small muscular branches of the pulmonary arterial tree are primarily concerned in determining pulmonary vascular resistance. Therefore, the study of their morphologic eharacteristics is of paramount importance for understanding the genesis and significance of high altitude pulmonary hypertension. This forms the subject of the present report.

Material and Method Two autopsy series are studied: one from peo-ple born and living permanently in high altitude areas of the Peruvian Andes (from 11,300 to 14,300 feet); the other, used as control, from people born and living at sea level. Subjects of approximately matching ages were considered in both series.