RATIONALE: Allergen exposure in sensitised asthmatics interacts with viruses in increasing the risk of asthma exacerbation. OBJECTIVES: To evaluate the use of house dust mite impermeable bedding on severe asthma exacerbations in children. METHODS: We randomized mite-sensitised asthmatic children (3-17 years), following an emergency hospital attendance with an asthma exacerbation, to receive mite-impermeable (Active) or control (Placebo) bed encasings. MEASUREMENTS AND MAIN RESULTS: Over a 12-month intervention period the occurrence of severe asthma exacerbations were investigated. Of 434 asthmatic children who consented, 286 (mean age 7.7 years, 65.8% male) were mite sensitised and 284 were randomised (146 Active; 138 Placebo). At 12 months, significantly fewer children in the Active group had attended hospital with an exacerbation compared to the Placebo group (36/123 [29.3%] versus 49/118 [41.5%], p=0.047). In the multivariable analysis, the risk of emergency hospital attendance was 45% lower in the Active group (Hazard Ratio 0.55[95%CI, 0.36-0.85], p=0.006) compared with the Placebo group. The annual rate of emergency hospital attendance with exacerbations was 27% lower in the Active compared with the Placebo group, but this did not reach significance (Estimated marginal mean [95% CI]: Active 0.38[0.26-0.56] vs Placebo 0.52[0.35-0.76], p=0.18). No difference between the groups in the risk of prednisolone use for exacerbation was found (Hazard Ratio 0.82[0.58-1.17], p=0.28). CONCLUSIONS: Mite-impermeable encasings are effective in reducing the number of mite sensitised asthmatic children attending hospital with asthma exacerbations, but not the number requiring oral prednisolone. This simple measure may reduce the health care burden of asthma exacerbations in children.