Fungus-induced obstructive airway disease in atopic individuals can be differentiated into two categories: first, uncomplicated asthmatic reactions due to high but transient exposure to fungal spores (fungal asthma), resulting in a TH2-type response with immunoglobulin E-mediated reactions and eosinophilic inflammation; and second, a more complex asthmatic reaction due to colonization of the mucus-epithelial surface by virulent protease-producing fungi. The latter condition stimulates as exaggerated immunological response including all subclasses of antibodies directed against the microorganism and an intense eosinophilic infiltrate of the airways. The authors propose that the exaggerated inflammatory response in allergic bronchopulmonary fungosis damages epithelial cells and underlying tissue cells, resulting in inefficient elimination of the microorganisms and damage to matrix proteins of the lung tissue by proteases released by both the fungi and degranulating eosinophils. The positive effects of corticosteroids in the treatment of allergic bronchopulmonary aspergillosis probably results from the dampening of the inflammatory response and an increase of the efficiency of killing the fungi. Sensitization to fungi is high in childhood and declines rapidly with age, suggesting that younger children may be less proficient in clearing fungi from the airways. We propose that insufficient treatment of fungal asthma may result in damage to the bronchial mucosa and formation of bronchiectasis.