Acute mountain sickness (AMS) has long been recognised as a potentially life-threatening condition afflicting otherwise healthy normal individuals who ascend rapidly to high altitude where the partial pressure of oxygen (pO2) in the air is reduce. The symptoms of AMS (e.g. headache, poor appetite and nausea, fatigue and weakness, dizziness or light-headedness and poor sleep) are probably a consequence of disturbances in fluid balance brought about by severe tissue hypoxia. AMS can be prevented by an adequately slow ascent, which is the best method, but for those with limited time there are several drug therapies that provide a relatively good protection. Acetazolamide (250 mg twice daily or 500 mg slow release once daily), taken before and during, ascent is probably the treatment of choice; it improves gas exchange and exercise performance and reduces the symptoms of AMS in most individuals. Dexamethasone (4 mg, 4 times daily) is more of value for short term treatment or prevention, and should never be used for more than 2 to 3 days. Prophylactic use of progesterone looks promising, but more studies are required.