Alterations in psychological mood, personality, behaviour and cognitive functioning associated with altitude have been recognised for many years. Psychological and behavioural changes resulting from the effects of hypoxia often include increases in euphoria, irritability, hostility and impairment of neuropsychological functions such as vision and memory. There is limited research to indicate that some decrements may persist for up to a year, or longer, after return to lower elevation. However, generalisations of the effects of altitude on mood, behaviour and cognitive function are complicated by differences between studies including the specific tasks or dimensions measured, methods of measurement, degree of altitude, duration of exposure and types of participants. There also appear to be wide individual differences in the response to altitude sojourns. Investigations designed to measure symptoms and mood changes during actual mountain climbs are needed, since most of the systematic research has been conducted in altitude chambers. The extent to which these simulations can be generalised to actual altitude settings remains to be documented.