Nonspecific airway hyperresponsiveness is a fundamental characteristic of patients with asthma and can be influenced by several stimuli. In nine patients with asthma and an isolated allergy to house dust mite, the variation of natural exposure to the house dust mite allergen Der p I and the corresponding changes in nonspecific airway hyperresponsiveness were followed up for 1 year. The concentration of Der p I in floor dust from living rooms and bedrooms (as a measure of exposure) reached maximum levels in late summer and the beginning of autumn (August to October), whereas the lowest levels were found during the months of March to May (delta Der p I = +2.31 micrograms/gm and -1.33 micrograms/gm respectively, both compared with the year average). Airway hyperresponsiveness (as measured by the provocative concentration of histamine causing a 20% fall in forced expiratory volume in 1 second [PC20] threshold) also showed a seasonal variation, with most severe hyperresponsiveness during the months of August to November, almost the same period in which the exposure to house dust mite allergens reached maximal levels (delta PC20 histamine = -1.47 mg/ml in November vs +1.79 mg/ml in March, both compared with the year average). Our results support the view that seasonal changes of exposure to environmental allergens such as house dust mite allergens will have an effect on the level of airway hyperresponsiveness in patients with allergic asthma.