We report spirometry and radiographic data on eight normal male human subjects during prolonged graded altitude exposure to as high as 8,848 m above sea level in a hypobaric chamber. We found a significant and progressive drop in FVC by 14 ± 3% over 40 days, which resolved slowly during the first 48 h after descent. With altitude, midrange forced expiratory flow (FEF 25-75 ) increased by 82 ± 3%, probably because of reduced air density. FEV 1 , however, did not change. Chest radiographs on subjects taken 2 h after descent to sea level showed a pattern of pulmonary artery enlargement and interstitial edema. These data suggest that increased pulmonary blood volume and edema may be causes of the restricted pulmonary function pattern.