Chronic mountain sickness (CMS) or Monge's disease is a clinical entity observed among residents at altitude characterized by polycythaemia increased above the physiological level due to altitude adaptation. From correlation studies of haematocrit with ventilation rate of healthy and diseased native high altitude residents (4,540 m) it was found that CMS is a clinical manifestation of aging. The higher the altitude the sooner the symptoms of excessive polycythaemia will become evident. © 1975 Swets & Zeitlinger B.V.