It was found that several foreign experts recruited for an irrigation project in southern Peru could not cope with their work at an altitude of 3 200 m. When we analyzed the haematological acclimatization to high altitude in 109 adult males, we found that the moderate differences in haematocrit level (0.9) and haemoglobin concentration (0.2 g/100 ml) between male smokers and non-smokers at low altitude were almost quadrupled at 3 200 m height. The haemoglobin concentration increased in both groups during the first month, after which it remained constant. Of 51 Swedish employees, 25 smokers and 26 non-smokers, 14 were obliged to terminate their contracts. All 10 who had failed to cope with their work for other than purely medical reasons were smokers. This group of smoking men also had an overconsumption of alcohol. We therefore conclude that life style determines long-term acclimatization to high altitude.