Objective.—To compare altitude responses of 2 ultraendurance athletes and 2 nonathletes during a 2-week expedition on Denali (Mount McKinley).

Methods.—The severity of acute mountain sickness (AMS) symptoms (Lake Louise AMS guidelines) and pulmonary function parameters (forced vital capacity, forced expiratory volume in 1 second, peak expiratory flow) as well as resting heart rate and arterial oxygen saturation measurements were taken during the climb. Baseline measurements were made at 375 m, and field tests were performed at altitudes of 2200 m, 2400 m, 3000 m, 3400 m, 4100 m, 4300 m, and 10 m.

Results.—Nonathletes reported moderate AMS symptoms at altitudes up to and including 3000 m, whereas ultraendurance athletes reported moderate AMS symptoms at altitudes above 3000 m. Considerable daily variation existed in pulmonary function measures within and between groups; however, the largest shift from baseline and between groups occurred at 3000 m where ultraendurance athletes had increased and nonathletes had decreased peak expiratory flow and forced vital capacity. Resting heart rate increased and arterial oxygen saturation decreased with altitude.

Conclusions.—Highly aerobically fit individuals may be more susceptible to delayed and more prolonged onset of AMS than are moderately fit individuals. Pulmonary function, although highly variable, also may be dissimilar between these groups.