BACKGROUND: Improvement of allergic asthma is seen at high altitude partly because of low concentrations of allergen, especially house dust mite. To investigate the effect of a hypoallergenic environment (Davos, 1560 m) on airways inflammation, the changes in bronchial hyperresponsiveness measured with methacholine and adenosine 5'-monophosphate (AMP), blood eosinophils, eosinophil cationic protein (ECP), and serum IgE were studied. METHODS: In 16 allergic asthmatic children tests were performed on admission and after one month. Medication was kept unchanged during the month of investigation and the patients performed peak expiratory flow (PEF) measurements twice daily. RESULTS: After one month at high altitude a considerable improvement was seen in the provocative concentration of AMP causing a 20% fall in forced expiratory volume in one second (PC20 AMP), but not with methacholine. There was also a reduction in total blood eosinophils and ECP. No change in serum IgE was observed. Peak flow variability decreased. CONCLUSIONS: After one month at high altitude a reduction in airways inflammation occurs. The results indicate that AMP responsiveness is a more accurate marker of disease activity in relation to inflammation in asthma than methacholine. The benefits of allergen avoidance at high altitude have important clinical implications for children with allergic asthma.