We measured lung mechanics in seven young adults at 90 m and again after 9-30 days of exposure to 5,366 m on the summit plateau of Mt. Logan. Increases in peak expiratory flow (by 20%, P less than 0.05) and decreases in total pulmonary resistance (by 30%, P less than 0.02) were consistent with the reduced gas density at high altitude. However, forced expiratory volume in 1 s decreased in four of the seven subjects, suggesting an increase in resistance of small airways or decrease in lung recoil associated with hypoxia. Increase in total lung capacity (TLC), as measured by helium dilution, occurred in each subject and by over 2 liters in two of the subjects (mean increase 17.9%, P less than 0.01). With the increased TLC's taken into account, there were still leftward shifts in the static expiratory pressure-volume (PV) curves with abnormally low lung recoil at 60% TLC, accounting for disproportionately large increases in residual volume (RV/TLC increased from 0.31 to 0.44, P less than 0.005). Prolonged exposure to high altitude is associated with increases in lung volume and shifts in PV curves indistinguishable from those previously described during exercise-induced asthma and during acute isocapnic hypoxia.