Cerebral edema is swelling of the brain which can occur as the result of a head injury, cardiac arrest or from the lack of proper altitude acclimatization.

Symptoms of cerebral edema include headaches, decreased level of consciousness, hallucinations, psychotic behaviour, memory loss and coma. If left untreated, it can lead to death.

High Altitude Cerebral Edema (or HACE) is a severe (usually fatal) form of altitude sickness. HACE is the result of swelling of brain tissue from fluid leakage. Symptoms can include headache, loss of coordination (ataxia), weakness, and decreasing levels of consciousness including disorientation, loss of memory, hallucinations, psychotic behavior, and coma. It generally occurs after a week or more at high altitude. Severe instances can lead to death if not treated quickly. Immediate descent is a necessary life-saving measure (2,000 - 4,000 feet). There are some medications (e.g. dexamethasone) that may be prescribed for treatment in the field, but these require proper medical training in their use. Anyone suffering from HACE must be evacuated to a medical facility for proper follow-up treatment. A gamow bag can sometimes be used to stabilize the sufferer before transport or descending.

Climbers may also suffer high altitude pulmonary edema (HAPE), which affects the lungs. While not nearly as life threatening as HACE in the initial stages, failure to descend to lower altitudes or receive medical treatment can also lead to death.