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Background and Aims: Seasonal allergy is an interesting model to study the pathophysiological mechanisms involved in allergic inflammation. However, experimental allergen exposure is easier to perform and standardise. The primary aim of this study was to compare the inflammatory responses to high-dose bronchial challenge and natural exposure during birch pollen season. The second aim was to compare the responses of patients with allergic rhinitis and allergic asthma, respectively to both types of allergen exposure., Methods: Fifteen birch pollen-allergic patients (seven with asthma and eight with rhinitis) and five healthy individuals were studied during pollen season and after challenge with birch allergen. Symptoms, medication and peak expiratory flow rate (PEFR) were recorded, and blood samples, spirometry and induced sputum were analysed during season and after challenge., Results: Patients with allergic asthma demonstrated a greater bronchial responsiveness to bronchial provocation with birch allergen than patients with rhinitis (P = 0.04) whereas no difference was found regarding nasal challenge. No significant association was found between the level of responsiveness and the inflammatory response after seasonal exposure. Seasonal exposure was related to a more marked systemic inflammatory blood-eosinophil increase than bronchial challenge [(median) (0.25 vs 0.11 x 109/L, P = 0.03)] and after nasal challenge, respectively [(median) (0.25 vs 0.04 x 109/L, P = 0.003)]. A significant correlation in eosinophil cationic protein in induced sputum was found between the experimental and seasonal exposure (rho = 0.62, P = 0.02)., Conclusions: Bronchial allergen challenge with inhalation of birch pollen gives a similar inflammatory response in the airway but less systemic inflammation than seasonal exposure in birch pollen allergic patients with asthma and rhinitis.