Since symptoms of Bronchopulmonary Aspergillosis are often identical to bacterial infections, the diagnosis is difficult to make. The disease presents with wheezing, pulmonary infiltrates, and bronchiectasis. The most important diagnostic parameters are asthmatic symptoms with obstruction, positive prick test, elevated total IgE, specific IgE and IgG to Aspergillus fumigatus, eosinophilia and radiological findings. Aspergillus fumigatus acts as an allergen Ig-E mediated allergy. Pathophysiological it is assumed that there are two different mechanisms of allergic inflammation. First, there is a direct effect of Aspergillus fumigatus proteases in the alveolar and bronchial epithelium with release of proinflammatory cytokines (IL-8, IL6, MCP-1) and consecutive chemotaxis of inflammatory cells. Second a CD4+ Th2 response with release of IL-4, IL-5 and IL-13. Recently published studies suggest that Aspergillus spores cause the TH2-dependent inflammation directly. So-called Chitinases (part of innate immunity) induce massive IL-13 stimulation. Induction of chitinase activity (CHIT1) leads to an increased remodeling of the lung. It is currently unclear, to which extent Aspergillus-triggered bronchial inflammation in patients with CF is relevant.