Acute Mountain Sickness (AMS) was studied using a questionnaire for trekkers climbing a 5400 m high pass in the Nepalese Himalaya. Over 8 days, 500 questionnaires were issued to 530 trekkers on the way to the pass. A total of 371 questionnaires was returned, and 353 were used for analysis. Trekkers with scores over 0.7 for AMS-C (cerebral ESQ-III score) and/or 0.6 for AMS-R (respiratory ESQ-III score) were considered to suffer from AMS. The overall prevalence of AMS was 63%; the prevalence of AMS-C was 43% and of AMS-R was 57%. AMS was positively correlated with rate of ascent and negatively correlated with pre-trek acclimatization. Women suffered more frequently and more seriously from AMS. Body mass index (weight/length2) was significantly correlated with AMS in men. No significant association was found between AMS and age, smoking habits, use of oral contraceptives, previous high altitude experience, special pre-trek training, size of trekking party or whether the trek was agency- or self-organised. Of trekkers, 80% had elementary knowledge of the diagnosis and treatment of AMS.