Acute mountain sickness, high-altitude cerebral edema, and high-altitude pulmonary edema are illnesses associated with acute exposure to altitudes greater than 8000 ft. Although usually self-limiting problems, they can be severe and life threatening. Gradual ascent to allow acclimatization can lessen or prevent symptoms. Acetazolamide is the drug of choice for pharmacologic prophylaxis; descent to lower elevation is the definitive treatment for altitude illness. Individuals who have chronic cardiac, respiratory, or certain other medical conditions may require supplemental oxygen and should be aware of restrictions regarding high-altitude exposure.