Acute mountain sickness (AMS) is usually a benign and self-limited illness that befalls previously healthy individuals who ascend rapidly to high altitude without sufficient acclimatization. In its more severe forms, AMS can progress to a life-threatening condition in which pulmonary or cerebral edema can occur singly or in concert. High altitude cerebral edema (HACE) is a little-known clinical entity that manifests itself by a perplexing array of both generalized and localized neurological symptoms and signs. Furthermore, the development of HACE in climbers offers a unique experimental situation in which to examine the effects of hypoxia on the central nervous system. The epidemiology and clinical picture of HACE are reviewed. In addition, the pathology and predominant pathophysiological mechanisms postulated to explain HACE are examined, and the present recommendations for the prevention and treatment of this dangerous and unusual form of brain swelling are discussed.