Acute mountain sickness is an illness that can affect mountain climbers, hikers, skiers, or travelers at high altitude (typically above 8,000 feet or 2,400 meters).

Acute mountain sickness is brought on by the combination of reduced air pressure and lower oxygen concentration that occur at high altitudes. Symptoms can range from mild to life-threatening, and can affect the nervous system, lungs, muscles, and heart.

In most cases the symptoms are mild. In severe cases fluid collects in the lungs (pulmonary edema) causing extreme shortness of breath. This further reduces how much oxygen enters the bloodstream and reaches vital organs and tissue. Brain swelling may also occur (cerebral edema). This can cause confusion, coma, and, if untreated, death.

The chance of getting acute mountain sickness increases the faster a person climbs to a high altitude. How severe the symptoms are also depends on this factor, as well as how hard the person pushes (exerts) himself or herself. People who normally live at or near sea level are more prone to acute mountain sickness.

Approximately 20% of people will develop mild symptoms at altitudes between 6,300 to 9,700 feet, but pulmonary and cerebral edema are extremely rare at these heights. However, above 14,000 feet, a majority of people will experience at least mild symptoms. Some people who stay at this height can develop pulmonary or cerebral edema.