In recent years increasing attention has been devoted to a group of pulmonary diseases characterized by a host reaction to the inhalation of various organic dusts. Thermophilic actinomycetes have been implicated as respiratory allergens in bagassosis and farmers' lung1,2 while avian and animal proteins have been found to be etiologically important in pigeon breeders' disease and pituitary snuff-takers' lung.3,4 Although these disorders are well-known in this country, reports of a similar but more complex syndrome, allergic bronchopulmonary aspergillosis, have come primarily from Great Britain.5 The reason for this is unclear since spores of the causative fungus, Aspergillus fumigatus (Fig 1) are frequently inhaled by the population at large and are a relatively common cause of allergic bronchial asthma in the United States.6,7 This report describes two patients with allergic bronchopulmonary aspergillosis and evaluates the role of immunologic tests in the diagnosis