Although the ability of Aspergillus organisms to colonize the respiratory tract in patients with cystic fibrosis (CF) is well recognized, the contribution of Aspergillus to the disease process is poorly understood. Using sera from 147 CF patients (age 5 to 43 yr), we measured IgE antibody (ab) to Aspergillus fumigatus and five common inhalant allergens with a radioallergosorbent test (RAST). Total IgE levels and IgG ab to radio-labeled Asp f I, an allergen purified from A. fumigatus and a potent inhibitor of protein synthesis, were also measured. Thirty (20%) of the patients had IgE ab to A. fumigatus, and 22 (15%) of these patients had developed total IgE levels > or = 400 IU/ml, raising the consideration of a diagnosis of allergic bronchopulmonary aspergillosis (ABPA). Five of the 22 patients developed these IgE responses by age 5 yr and 14 by age 10 yr. The proportion of patients with IgE ab to one or more of the other allergens tested was not significantly different from that of control subjects without respiratory symptoms. A striking proportion (84%) of CF sera contained IgG ab to Asp f I, compared with 6% of sera from control patients and 20% of sera from allergic children with asthma (n = 25), only one of whom had IgE ab to A. fumigatus. In an examination of additional sera from young CF patients, IgG anti-Asp f I ab was detected in 41% of these sera from patients 5 yr of age or older, increasing to 98% of 89 sera from patients older than age 10.(ABSTRACT TRUNCATED AT 250 WORDS)