The symptoms of mountain sickness extend from slight malaise to severe clinical disease. From our own observations in the Himalayas the most serious symptoms appear at night, i. e. at less than the meximum height achieved and when physically at rest. The etiology is not completely clear; the reduced oxygen partial pressure at a height plays a principal role. Prophylactic measures are empirical, scientific methods are being worked on. Treatment is symptomatic, causal therapy consists of rapid removal from the height and oxygen respiration.