The effect of intermittent exposure to high altitude (4200 m) hypoxia on symptomatology of acute mountain sickness (AMS) was observed, and the relationship between AMS and stress hormone excretion was examined among shift workers at the United Kingdom Infrared Telescope (UKIRT) facility and a control group of sea level residents. Some of the shift workers experienced AMS during their first day at altitude, but had recovered after 5 days residence at altitude. Upon acute exposure to altitude, none of the shift workers reported severe cases of AMS after 5 days residence at sea level, but some reported severe cases after 45 residence at sea level, thus providing some evidence for a 'carry over' of acclimatization for a 5 day period. Reported symptomatology at sea level was predictive of 24 h. excretion rates of adrenaline and 17-hydroxycorticosteroids at high altitude. No good predictors of symptomatology of AMS at high altitude were found using sea level measures alone.