Allergic bronchopulmonary aspergillosis (ABPA) is a syndrome seen in patients with asthma and cystic fibrosis. It is characterized by chronic colonization of the airways with a ubiquitous fungus, Aspergillus fumigatus. The clinical expression of ABPA results from the complex interaction of chronic colonization of the airways with A fumigatus, host factors allowing this colonization, and the host's genetically determined immune response. Clinically the syndrome is characterized by recurrent episodes of wheezing, mucus production, pulmonary infiltrates, and elevated levels of serum IgE. Many patients develop central bronchiectasis, and a subset will go on to endstage fibrotic lung disease. It is thought that treatment will prevent this progression. The mainstay of therapy remains oral corticosteroids.