Acute mountain sickness (AMS) is a neurological syndrome characterized by headache, nausea, vomiting, insomnia and ataxia and usually observed within 4 to 96 hr of abrupt ascent to high altitude. The authors wished to see if the electroencephalogram (EEG) reflected the severity of AMS or could be used to determine which people were at risk of developing AMS. Seven healthy male subjects were studied at sea level and at 5,360 m and the severity of AMS was quantified twice daily on a 0 to 24 point scale. The EEG was recorded postprandially at rest and during voluntary hyperventilation: slow wave activity was quantified using a time averaged frequency analyzer, and a frequency measured directly from the EEG recording. Although in 6 of 7 subjects the increase in EEG slow wave activity with hyperventilation at sea level was inversely related to the severity of AMS, the 7th subject developed the severest AMS and the most slow wave activity during hyperventilation. All subjects' rest EEG's showed an increase in frequency on arrival at 5,360 m, with a decrease in a frequency after 48 hr to less than sea level values. The subjects with the more severe AMS had higher α frequencies at sea level and on arrival at 5,360 m, and showed larger decreases in α frequency at altitude. Change in α frequency was correlated with change in arterial PO2, but no PCO2 or pH. Breathing 100% O2 for 30 min, however, did not alter α frequency at altitude. Slow wave activity increased at altitude in all subjects, but was not correlated with severity of AMS. Thus measurement of α frequency at sea level may be of value in the prediction of AMS and slowing of the α rhythm at altitude appears to reflect the severity of AMS.