Background: Exposure to inhaled allergens is a pathogenetic factor in allergic asthma. However, most studies that previously looked at air cleaning devices have shown little or no effect on patients with perennial allergic asthma. Aims and objectives: This study aims to investigate a novel treatment, TLA, using temperature controlled laminar airflow with a very low particle concentration directed to the breathing zone of teenagers and adults with allergic asthma during night sleep. The hypothesis is that the decreased allergen exposure during the night will have an effect on bronchial inflammation and quality of life. Method: Patients are randomized to start with active or placebo treatment for 10 weeks. All patients receive both active and placebo treatment with unfiltered air, with a 2-week wash-out period in between treatments. Maintenance treatment with inhaled corticosteroids is unaltered during the trial period. Health related quality of life (miniAQLQ)is the primary effectiveness measure. Exhaled nitric oxide (FeNO) and spirometry are also investigated.