Acute high-altitude illness usually occurs at elevations of 8,000 ft or more, is more common than is generally appreciated and is often confused with other disorders. The acute forms of high-altitude illness include acute mountain sickness, high-altitude pulmonary edema, high-altitude cerebral edema and high-altitude retinal hemorrhage. These illnesses often occur simultaneously and in severe forms can be fatal. Gradual ascent and acetazolamide help prevent acute mountain sickness and high-altitude pulmonary edema. The definitive treatment for all forms of high-altitude illness is descent to a lower altitude.