Chronic mountain sickness, which affects permanent residents of high altitudes, is the outcome of a progressive loss of ventilatory rate which naturally occurs with age and resulting in excessive hypoxemia and polycythemia. A theoretical model predicts the progressive failure of homeostatic control of the hemoglobin concentration when the values increase above those found at sea level. This is confirmed by lack of feedback mechanism between high altitude erythrocytosis and serum erythropoietin. The results of epidemiological studies are in agreement with the physiological findings. In a male population living at 4,300 m, an increase with age of the prevalences of excessive erythrocytosis (Hb> 213 g/l), blood oxygen saturation< 83%, headaches and a high score of symptoms of chronic mountain sickness has been found. The studies suggest the possibility that in addition to an accentuated hypoxemia, the excessive erythrocytosis may also result from an overreaction of the bone marrow to a fixed level of hypoxemia in ageing individuals.