Pathologic data on chronic mountain sickness are scarce due to the fact that the disease is ameliorated or cured by descent to a low altitude. A case of chronic mountain sickness occurring in a woman of 48 yr at Cerro de Pasco (4,300 m above sea level) is described. The necropsy findings are compared with the limited pathologic observations reported by others. It is apparent from the findings that in fatal cases the main changes are located within the pulmonary circulation. So far histologic studies have been reported only in cases of the secondary form of chronic mountain sickness. The basic pathology of the primary form (Monge's disease) remains to be defined.