Asthma is now considered as an inflammatory airway disease. There is evidence that allergen avoidance reduces clinical symptoms in atopic asthma. We investigated the effect of a month's stay in the hypoallergenic environment of Davos, Switzerland (1560 m) which is relatively free of house dust mite (HDM) on changes in bronchial hyperresponsiveness (BHR), using the challenge tests of adenosine 5'-monophosphate (AMP), exercise and methacholine to test for BHR. Thirteen asthmatic children with an allergy to HDM participated in the study. We measured BHR on admission to the Davos Asthma Center and after 1 month in the house dust-free environment. The medications used by the patients at the time of admission were kept unchanged during this month. No significant difference in BHR was found to methacholine challenge after a 1-month stay at high altitude (P > 0.05). By contrast, the response to AMP was significantly different as indicated by displacement of the dose-response curve to the right by 2.15 doubling concentrations (P = 0.005). We also observed a significant difference in response to exercise (P = 0.03). These results indicate that a month's stay in a hypoallergenic environment caused a reduction in BHR to AMP and exercise, but not to methacholine. In addition, the results support the concept of differences in trigger mechanisms for BHR, and that responses to a methacholine challenge are not the same as responses to an exercise challenge. The observed reduction in BHR in asthmatic children to the indirect bronchial stimuli of AMP and exercise suggest reduced airway inflammation following avoidance of house dust aeroallergens. AMP and exercise challenges may therefore be better indicators of asthmatic airway inflammation than the direct stimulus of methacholine.