Background: Acute Mountain Sickness (AMS) refers to signs and symptoms associated with hypobaric hypoxia. Its reported incidence is highly variable. Aim: To determine the incidence of AMS symptoms and severity at 3,500 and 4,250 m above sea level. Subjects and methods: A population of 362 soldiers without former exposure to altitude was studied. AMS symptoms, were assessed by an extensively used standard questionnaire (Lake Louise), applied 36-72 hours after exposure to high altitude. Results: A group of 200 recruits ascended to Putre (3,500 m) and a second group (162) ascended to Alto Pacollo (4,250 m). The incidence of AMS was 28% and 60% respectively (p<0.05). Headaches and sleeping difficulties were the most frequent symptoms at both altitudes. Furthermore, severe digestive problems and dizziness were described in a high proportion of individuals at both 3,500 and 4,250 m. Conclusions: The prevalence of AMS in this study is similar to that reported elsewhere at equivalent altitudes (Rev Méd Chile 2001; 129: 166-72).