Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity lung disease caused by bronchial colonization with Aspergillus fumigatus that affects approximately 10% of patients with cystic fibrosis (CF). The diagnosis in CF patients is difficult because the cardinal symptoms of ABPA occur frequently in CF, ie, pulmonary infiltrates and wheezing, as well as the frequent colonization with A fumigatus that leads to humoral reactivity. If left untreated, ABPA leads to bronchiectasis and pulmonary fibrosis. The pathogenesis of ABPA seems to be a prolonged asthmatic late-phase reaction orchestrated by CD4+ Th2-like T cells in response to persistent pulmonary A fumigatus allergen exposure. Thus, polyclonal and A fumigatus-specific IgE antibodies (and IgA and IgG) and blood pulmonary eosinophilia are stimulated by Th2-derived cytokines such as IL-4 and IL-5. In addition, IL-4 would also promote pulmonary transendothelial migration of eosinophils, basophils, and lymphocytes via induction of cell adhesion molecules and their ligands. IgE mast cell interactions would also contribute to the bronchial reactivity and inflammation. Recent advances have begun to identify immunodominant A fumigatus allergens. Evaluation of the quantity of IgE antibodies (and IgA and IgG) and T-cell cytokine responses to specific A fumigatus allergens should aid in the diagnosis and immunopathogenesis of ABPA, especially in CF patients.