BACKGROUND The in vitro diagnostic value of recombinant allergens depends on the correlation between allergen specific IgE in serum and ability of the protein to elicit immediate type allergic reactions in sensitized individuals. Objective The study was carried out to evaluate the reliability of recombinant Asp f 3 (rAsp f 3)-based serological determinations in ELISA and ImmunoCAP compared to skin-test responses in A. fumigatus sensitized subjects. METHODS Patients suffering from allergic bronchopulmonary aspergillosis (ABPA, n=11) or allergic asthma with A. fumigatus sensitization (n =8) and healthy control subjects (n=4) were investigated. Intradermal skin tests (IDT) and allergen-specific serology were performed using rAsp f 3 protein. RESULTS All 11 patients with ABPA and five out of eight A. fimigatus-sensitized asthmatics without ABPA exhibited a type I skin reaction. All rAsp f 3 skin test positive subjects showed relevant rAsp f 3-specific serum IgE levels compared with IDT-negative individuals who scored below the rAsp f 3-ImmunoCAP cut-off values of < 0.35 kU(A)/L. Analyses of rAsp f 3-specific immunoglobulins revealed significantly higher serum levels of IgG, IgG1, IgG4 and IgE antibodies in patients with ABPA compared with A. fumigatus-sensitized asthmatics and healthy controls. CONCLUSIONS rAsp f 3 represents a major allergen of A. fumigatus. It is recognized by 84% of asthmatic individuals sensitized to the fungus and elicits specific type I skin reactions. The skin test outcome strictly depends on the presence of rAsp f 3-specific IgE and the lack of false positive or false negative results comparing skin test outcome with specific serum IgE levels determined by ELISA or rAsp f 3-ImmunoCAP suggests the possibility to rely on serological data obtained with recombinant allergens to diagnose sensitization to A. fumigatus.