The incidence of acute mountain sickness was determined by questionnaire in 454 individuals who attended week-long continuing medical education programs at ski resorts in the Rocky Mountains with base elevations of about 2000 m. As a control group, 96 individuals who attended continuing medical education programs at sea level in San Francisco completed similar questionnaires. Study subjects were classified as having acute mountain sickness when they reported three or more of the five possible cardinal symptoms: headache, insomnia, dyspnea, anorexia, and fatigue. Only symptoms with an intensity of at least grade 2 (moderate) out of 5 were analyzed. Acute mountain sickness-like symptoms occurred in 25% of subjects at 2000 m compared with 5% of subjects at sea level. The incidence of acute mountain sickness at 2000 m was greatest among subjects who had come from lower altitudes. Half of the subjects with symptoms took medication. The duration of symptoms was short, with 90% of all symptoms that were reported occurring in the first 72 hours. Acute mountain sickness is common at intermediate altitudes, and it is frequently severe enough to prompt self-medication.