The development of bronchial hyperresponsiveness (BHR) in asthma is considered to be caused by inflammation of the airway. In IgE-mediated allergy BHR is related to the occurrence of late phase reactions. We have previously shown that exposure to low doses of allergen can cause isolated late reactions. These findings are potentially of clinical importance, since exposure to low, subclinical allergen doses may lead to bronchial inflammation and increasing bronchial responsiveness without necessarily causing immediate bronchoconstriction. This study was performed to investigate whether repeated exposure to low doses of allergen could induce a change in BHR. The trial comprised two groups of five and eight patients with a history of allergic asthma. They were submitted to a series of allergen inhalations for 5-7 days. They were given the same low allergen dose (1-10 biological units) each day. Before and after the allergen exposure period histamine challenges were performed. After the week of allergen inhalation the bronchial responsiveness was increased in 11 of 13 patients.