High-altitude pulmonary edema (HAPE) is one of the dangerous symptoms of altitude sickness. It's the major cause of death related to high-altitude exposure, with a high mortality rate in the absence of adequate emergency treatment.
It occurs in about 1% of otherwise healthy mountaineers at altitudes typically above 2,500 meters (8,200 ft).
The initial cause of HAPE is a shortage of oxygen caused by the lower air pressure at high altitudes, but the mechanisms by which this oxygen shortage leads to HAPE are poorly understood. Two processes are believed to be important:
• Increased pulmonary arterial and capillary pressures (pulmonary hypertension) secondary to hypoxic pulmonary vasoconstriction.
• An idiopathic non-inflammatory increase in the permeability of the vascular endothelium.
More info at Wikipedia.