Chronic mountain sickness (CMS) is a clinical syndrome that occurs in natives or long-life residents above 2500m. It is characterized by excessive erythrocytosis (EE; women hemoglobin (Hb) ≥19gdl-1; men Hb ≥21gdl-1) and severe hypoxemia which gets worse at bedtime. The etiology of CMS is a decrease of the ventilatory function, which at high altitude produces hypoxemia, the basis of the EE. It exists in a primary form and in a secondary form that is associated with other conditions. The diagnosis of CMS is established primarily through medical review of symptoms and Hb level, which, when excessive, can be treated by bloodletting, medroxyprogesterone (in men), or acetazolamide. The clinical symptoms of CMS gradually disappear after descending to low altitudes and reappear after returning to high altitudes.