To study the long-term effects of avoiding domestic allergens, nine asthmatic patients who were allergic to dust mites lived in hospital rooms for two months or more. In all patients symptoms and early morning peak flows improved. In seven patients anti-asthma treatment could be reduced and it was possible to carry out repeated bronchial provocation with histamine. Five of these patients showed a progressive eightfold or greater increase in the concentration of histamine necessary to provoke a 30% fall in forced expiratory volume in one second (PD30). The increase in PD30 in the seven patients during their period of living in hospital was highly significant. Avoidance of important allergens seems not only to result in clinical remissions but in many cases also reduce bronchial hyperreactivity.