As more people enjoy the outdoors, high-altitude illness is increasingly becoming a problem that family physicians across the country must treat. High-altitude illness, which usually occurs at altitudes of over 1,500 m (4,921 ft), is caused primarily by hypoxia but is compounded by cold and exposure. It presents as one of three forms: acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE). But high-altitude illness can have many other manifestations. Cardinal symptoms include dyspnea on exertion and at rest, cough, nausea, difficulty sleeping, headache and mental status changes. Treatment requires descent, and gradual acclimatization provides the most effective prevention. Acetazolimide is an effective preventive aid and can be used in certain conditions as treatment.