Long-term exposure to high altitude has been reported to impair cognitive functions, possibly resulting in an increased risk of mountain accidents. To assess the modification of cognitive functions during acclimatisation to altitude, 17 climbers were studied at 5350 m a.s.l. by means of a neuropsychological learning test. The results clearly show that by extending the period spent at elevations above 5350 m to more than 15 days, the response to a memory task was significantly enhanced. The improvements resulting from acclimatisation were more evident in the organisation of information than in information storage. We suggest that inappropriate acclimatisation has a detrimental effect on cognitive functions and the resulting impairment may particularly affect the more demanding technical tasks.