In this report we described the findings in a case of Chronic Mountain Sickness, autopsied at Cerro de Pasco (4, 300 feet) and compared with the limited pathologic observations reportedby others. It is aparent that in fatal cases of Chronic Mountain Sicknessthe main changes are located at the level of the pulmonary circulation and are of the type that lead to Chronic Cor Pulmonale. So far histologic studies have been reported only in cases of secondary Chronic Mountain Sickness. The basic pathology in Primary Chronic Mountain Sickness or Monge’s disease remain to be defined. © 1973 S. Karger AG, Basel.