The aim of the study was to determine the factors affecting a climber's ability to ascend Mont Blanc using a number of variables collected at the Gouter Hut (3,817 m) before and after an attempted ascent on the Mont Blanc summit. Subjects (n=285) were tested at 3,817 m prior to their ascent of Mont Blanc. Maximum height ascended in the last 14 days was recorded. End tidal CO2, arterial oxygen saturation (SaO2), heart rate and respiratory rate were measured using a Capnograph (Nellcor Patrick NPB75). Acute mountain sickness (AMS) was assessed using the Lake Louise scoring system. Summit information is available for 216 subjects. None of the subjects who attained 4,000 m in the previous 14 days failed to reach the summit (P=0.04). Previous recent exposure to an altitude of 4,000 m resulted in faster ascent times to the summit than those who had not been above 3,000 m in the previous 14 days (4.02+/-0.6 vs. 4.46+/-0.8 h, P=0.009), higher SaO2 on arrival at the Gouter Hut on day 1 (88.6+/-5 vs. 86.3+/-6%, P=0.004) and lower AMS scores upon arrival at the Gouter Hut after the attempted ascent to the summit 2.5+/-1.8 versus 4.7+/-2.5 U (P=0.001), respectively. It is concluded that recent exposure to 4,000 m confers an advantage to those who wish to ascend a 4,800 m peak.