Altitude sickness.

Ten climbers were studied trekking from 1,950 to 4,650 m (6,500 ft to 15,500 ft) in the Nanda Devi Sanctuary of the Garwhal Himalaya. All developed altitude sickness, one seriously. Pulse, blood pressure, and peak flow rate were monitored daily in an attempt to predict the onset of altitude sickness. Prediction was uncertain though the one climber who became seriously cyanosed at 4,200 m (14,000 ft) had a consistently higher blood pressure than his colleagues.