The objective of this study was to examine how pulmonary ventilatory function, including response to bronchodilation, is related to altitude during high-altitude trekking. This cohort experiment consisted of multiple spirometric tests before and after bronchodilation in participants at baseline (1624 m) and at different altitudes (3404-4896 m) during a 2-week trek. The setting was in the Himalayas. Eleven men (ages 22-68 years) and eight women (ages 19-42 years) participated. Interventions were at altitudes of 1624 m to 5265 m; albuterol was administered via Rotahaler. Forced vital capacity (FVC) decreased by an average of 3.8% [95% confidence interval (CI) 1.6 to 6.0] per 1000-m altitude increment. Forced expiratory volume in 1 second (FEV1.0) decreased 3.7% (95% CI 1.9 to 5.5) per each 1000-m altitude increment. Maximal midexpiratory flow rate (FEF25-75%) decreased by 3.6% (95% CI 0.9 to 6.3) per each 1000-m altitude increment. Small, postalbuterol flow increases were present at baseline and at altitude. Ventilatory function returned quickly toward baseline upon descent. One trekker developed cough, dyspnea at rest, extreme weakness, rales, tachycardia, and oxygen desaturation to 71%. His ventilatory measurements did not differ significantly (p > 0.32) from the group means. We concluded that changes in some pulmonary ventilatory parameters (FVC, FEV1.0, and FEF25-75%) were proportional to the magnitude of altitude during a high-altitude trek. These were tolerated well and do not seem to relate to acute mountain sickness. A bronchodilator effect was not increased at altitude.