In brief: High-altitude pulmonary edema afflicts 1% to 2% of sojourners above 10,000 to 11,000 ft. It is at the severe end of a spectrum of high-altitude illnesses; symptoms can be subtle or overt and can rapidly progress to coma and death. The best prevention is acclimatization—slow, gradual ascent that allows the body to adapt to a hypoxic environment. The author reviews the signs and symptoms and how to recognize them, the pathophysiology, and the prevention and management of this avoidable, but potentially fatal, condition.