Before specific therapy, such as oral corticosteroids, can be commenced it is essential to distinguish full-blown allergic bronchopulmonary aspergillosis (ABPA) from allergy to A. fumigatus in patients with cystic fibrosis (CF). For this purpose we have evaluated the diagnostic value of recombinant A. fumigatus allergen I/a (rAsp f I/a)-specific serology in 55 patients with CF. Based on clinical presentation and laboratory data, 10 CF patients had ABPA, 27 had Aspergillus allergy, and 18 were not allergic to A. fumigatus (CF control group). The serologic assays revealed a 10-fold increase in rAsp fI/a-specific IgE, a 5-fold increase in rAsp fI/a-specific IgG1, and a 4-fold increase in rAsp fI/a-specific IgG4 antibodies in ABPA patients compared with the Aspergillus allergy and CF control groups. Sera from 11 CF patients were analyzed without knowledge of their clinical state or diagnosis of ABPA. All ABPA patients (4 of 11) were accurately identified. We conclude that rAsp fI/a-specific serology is a highly sensitive and specific test that can be used to identify ABPA reliably in CF patients.