Medicine at high altitude provides important insights into the acute and chronic effects of hypoxia. Acute mountain sickness (AMS) is a common syndrome occurring after acute ascent to over 2,500 m and is caused by increased capillary permeability. A number of factors have been identified that increase the risk of AMS, in particular exercise. Avoiding rapid ascent, undue exercise and the use of acetazolamide are useful preventative measures but severe symptoms may require oxygen, dexamethasone and descent. Acute mountain sickness is usually self-limiting but may progress into the serious syndromes of pulmonary and cerebral oedema. Acclimatisation and adaptation are important for workers and residents at high altitude and the improvement seen in maximum exercise has been incorporated into some training schedules for endurance athletes. Chronic and subacute high-altitude diseases largely result from polycythemia and pulmonary hypertension.