Background House dust mite (HDM) allergy is a frequent cause of allergic asthma among children. Children spend 14 hours of their time indoor everyday in aberage, where they are exposed to different components in the indoor air. These components are children with asthma and HDM allergy specially sensitive towards. Reducing this exposure may improve the asthma control in these children. Previous studies give conflicting information on the effect of mechanical ventilation on asthma control in children. Objectives We aim at investigating whether mechanical ventilation is capable of improving indoor air quality and thereby asthma symptoms and quality of life, in children with house dust mite allergy and asthma. Materials and Methods Randomized double-blind placebo-controlled intervention study, including 80 children from 3 Danish Pediatric outpatient clinics, with: Verified asthma, requiring a minimum of 400 µg of inhaled steroid daily Verified HDM allergy Verified exposure to HDMallergene No pet- or birch allergy A ventilation system will be placed in to the child’s bedroom, ventilating the room with a rate of 2-3 air changes per hour for 9 months during the winter season. We monitor indoor air quality in the home and health outcomes in the outpatient clinic every three months. Fig. 1 and 2. Primary outcome is reduction in minimal effective dose of inhalation steroid. Secondary endpoints….. Perspectives Asthma patients and their families rely on good evidence-based advice on behavior and design of housing. Indoor environment factors that trigger the disease must be controlled as well as possible. The results of this project will be a significant contribution to the potential recommendations regarding the effect of ventilation on indoor air quality and asthma control of HDM allergic children Acknowledgments RealDania, Landsbyggefonden, The Danish Building Defects Fund.