Acute altitude exposure may lead to acute mountain sickness (AMS). Increased awareness of altitude-related health hazards in trekkers may accompany a decrease in AMS prevalence. We compared awareness and AMS prevalence in trekkers in two cohorts on an altitude trek up to 5400 m and assessed risk factors for AMS by repeating an observational cohort study 12 yr after an initial study. Questionnaires in English were distributed to two cohorts of 500 trekkers in 1986 and 1998. All trekkers over a several day period were asked to participate. Average participation rate was 62% (71% in 1986 and 53% in 1998). We found an increase in AMS awareness in trekkers from 80% to 95%, a decrease in AMS prevalence from 43% to 29%, and significant slower climbing profiles. We found no relationship between AMS and smoking habits, body mass index, oral contraception intake, or training status. By contrast, age was a strong independent risk factor inversely related to AMS. Subjects over 55 yr were 2.6 times less likely to suffer from AMS than subjects under 25 yr. Self-medication, including acetazolamide and analgesics, had increased importantly from 17% to 56%, and contraception intake in women had increased from 19% to 32%. In conclusion, in 1998 as compared to 1986, trekkers were older, climbed more slowly, had better awareness of altitude illness, used more medication, and suffered less from AMS.