Prevalence of acute mountain sickness in the Swiss Alps. OBJECTIVE--To assess the prevalence of symptoms and signs of acute mountain sickness of the Swiss Alps. DESIGN--A study using an interview and clinical examination in a representative population of mountaineers. Positive symptoms and signs were assigned scores to quantify the severity of acute mountain sickness. SETTING--Four huts in the Swiss Alps at 2850 m, 3050 m, 3650 m, and 4559 m. SUBJECTS--466 Climbers, mostly recreational: 47 at 2850 m, 128 at 3050 m, 82 at 3650, and 209 at 4559 m. RESULTS--In all, 117 of the subjects were entirely free of symptoms and clinical signs of acute mountain sickness; 191 had one or two symptoms and signs; and 158 had more than two. Those with more than two symptoms and signs were defined as suffering from acute mountain sickness. At 4559 m 11 climbers presented with high altitude pulmonary oedema or cerebral oedema, or both. Men and women were equally affected. The prevalence of acute mountain sickness correlated with altitude: it was 9% at 2850 m, 13% at 3050 m, 34% at 3650 m, and 53% at 4559 m. The most frequent symptoms and signs were insomnia, headache, peripheral oedema, and scanty pulmonary rales. Severe headache, vomiting, dizziness, tachypnoea, and pronounced pulmonary rales were associated with other symptoms and signs and therefore characteristic of acute mountain sickness. CONCLUSION--Acute mountain sickness is not an uncommon disease at moderately high altitude--that is, above 2800 m. Severe headache, vomiting, dizziness, tachypnoea, and pronounced pulmonary rales indicate severe acute mountain sickness, and subjects who suffer these should immediately descend to lower altitudes. 