The purpose of the study was to investigate determinants of acute mountain sickness after rapid ascent to high altitude. A total of 21 climbers were studied ascending from <1,200 m to Capanna Regina Margherita, a hut in the Alps at 4,559 m, within <24 h. During their overnight stay at 4,559 m, breathing patterns and ventilation were recorded by calibrated respiratory inductive plethysmography along with pulse oximetry. In the following morning, acute mountain sickness was assessed. Altogether, 11 mountaineers developed pronounced symptoms of acute mountain sickness (Lake Louise score > or =5) and 10 did not (controls). Compared to controls, subjects with acute mountain sickness had lower nocturnal oxygen saturation (mean+/-SD 59+/-13% versus 73+/-6%), higher minute ventilation (7.94+/-2.35 versus 6.06+/-1.34 L x min(-1)), and greater mean inspiratory flow, a measure of respiratory centre drive (0.29+/-0.09 versus 0.22+/-0.05 L x s(-1)). Periodic respiration was prevalent but not significantly different among the two groups (apnoea/hypopnea index 60.1+/-34.6 versus 47.1+/-42.6 events per h). The data suggest that pronounced nocturnal hypoxemia, which was not related to hypoventilation, may have promoted acute mountain sickness. Periodic breathing seems not to play a predominant role in the pathogenesis of acute mountain sickness.