Easily measurable variables capable of predicting altitude sickness, before symptom onset, would be beneficial for those requiring slower ascent or even medical attention. Unfortunately few such reliable factors exist. Our objective was to test respiratory function during ascent in the Nepali Himalaya, to see if any correlations with altitude sickness, expressed by Lake Louis scores, could be identified. Eleven healthy subjects (six male and five female, mean(SD) age 26(9.3) years) travelled from 1370m to 4200m, and spent 9 days at or above this altitude. Variables of lung function, including vital capacity, forced expiratory volume over one second (FEV₁) and peak expiratory flow were measured using a portable spirometer every-other morning before breakfast in the standing position. Subjects also completed the Lake Louis self-assessment questionnaire daily. Mean(SD) vital capacity and FEV₁ decreased from 5.3(1.4) L and 4.5(0.9) L at 1370m to 4.8(1.4) L and 4.2(1.0) L at 4200m, respectively (p<0.05), but did not correlate to Lake Louis scores. Mean(SD) peak expiratory flow was 9.4(2.3) L/s at 1370m and did not change during ascent. However once at 4200m it increased from 9.2(2.7) L/s to 9.6(2.4) L/s following the stay at altitude (p<0.05). Interestingly, an absolute change in peak expiratory flow at 4200m compared to 1370m, showed a high correlation to Lake Louis scores at 3700m and 4200m (r= -0.971; p<0.001). We conclude that peak expiratory flow values were closely related to signs of altitude sickness, and should be explored further for determining their predictive value.