To measure the association between changes in house dust mite (HDM) allergen Der p I exposure and changes in the severity of asthma, we re-analysed data from a clinical trial in which 34 HDM-allergic subjects with asthma (18 women, mean age 35 years) were followed for between 3 and 12 months. The concentration of Der p I in fine dust from the bed, the bedroom floor and the living room floor was measured at 3-monthly intervals along with assessment of subjects' spirometric function and airway hyperresponsiveness (AHR, measured by histamine inhalation test). Daily symptom scores, morning peak expiratory flow rate and peak flow variability were measured throughout the study period. The mean Der p I concentration in the bed at baseline was 25.4 μg/g (95% CI: 15.8–40.6). During the course of the study large within-subject fluctuations were observed in allergen concentrations and in the measurements of the severity of asthma. Changes in allergen concentration in the bed were significantly correlated with changes in AHR (P= 0.003) and symptom score (P= 0.04). Changes in allergen concentration in the living room floor were correlated with changes in symptom scores (P= 0.01). Although these correlations were significant the magnitude of the effect was relatively modest. We conclude that a large reduction in HDM allergen concentration, particularly in the bed, results in a modest reduction in AHR and improvement in symptoms in HDM-allergic subjects with asthma.