Numerous cases of acute mountain sickness (AMS) during trekking were reported to have been successfully treated with portable pressure chambers. The effect of early pressurization during acute altitude exposure in the Alps had not been previously studied. In order to test the hypothesis that an early pressurization of unacclimatized subjects for 3 h could prevent or delay the appearance of symptoms of AMS, 51 previously healthy subjects climbed from 1,030 to 4,360 m within 12 h. Upon arrival at 4,360 m, AMS scores (Lake Louise Consensus Questionnaire '91), oxygen saturation (SaO2), and heart rate (HR) were determined at rest. The subjects were then randomly divided in two groups; one group was pressurized to 200 mBar for 3 h while the other rested. AMS score, HR, and SaO2 were similar in both groups before treatment. AMS score had decreased (from 2.44 +/- 0.41 (S.E.) to 0.89 +/- 0.26, p < 0.05) and SaO2 had increased (from 75.22 +/- 1.32% to 79.07 +/- 1.27%, p < 0.05) in the treatment group 15 min after leaving the pressure chamber whereas the control group had unchanged AMS score (2.50 +/- 0.40 vs. 2.40 +/- 0.40, N.S.) and SaO2 (77.83 +/- 1.41 vs. 76.67 +/- 1.24, N.S.). The next morning, however, AMS score, HR, and SaO2 were similar for both groups. It is concluded that during acute ascent in the Alps, an early 3-h pressurization of unacclimatized subjects does slightly delay the onset of AMS but does not prevent the illness nor does it attenuate its severity upon appearance.