OBJECTIVE To compare a range of physiological responses to acute sea level hypoxia at simulated altitudes with the same physiological responses and acute mountain sickness (AMS) scores measured at altitude (similar to the simulated altitudes) during a 17 day trek in the Himalayas. METHODS Twenty men and women aged 18-54 years took part in the study. End tidal CO(2) tension (PETCO(2)) and saturated oxygen (SaO(2)) were measured using a capnograph. Observations made at sea level and some simulated altitudes were compared with those in the Himalayas. Pairwise correlations were used to examine the correlation between variables and regression, with 95% prediction intervals providing information on how well one variable could be predicted from another for a given subject. RESULTS There was only a significant correlation for a few comparisons. The 95% prediction intervals for individual SaO(2) values at a range of simulated altitudes were fairly wide going from +/- 4% to +/- 5%. All of the correlations between laboratory and Himalayan PETCO(2) values were not statistically significant. There was a significant correlation for the relation between SaO(2)and PETCO(2) at sea level for the laboratory data (r=-0.51; p=0.05). For the Himalayan data, there were significant correlations at Namche Bazaar (3450 m; day 3) (r=-0.56; p=0.01) and Dingboche (4300 m; day 6) (r=-0.48; p=0.03). The correlation between SaO(2) and PETCO(2) and AMS was generally poor. CONCLUSIONS The results indicate that limited information can be gained on a subject's response to altitude by assessing physiological variables at sea level and a range of simulated altitudes before the subject carries out a trek at altitude.