The diagnosis of allergic diseases is based on a combination of clinical history, physical examination, in vivo challenges, and in vitro test methods for the detection of allergen-specific IgE antibodies in the serum of patients. In spite of the fact that the determination of allergen-specific IgE in serum is widely used in daily clinical practice, one should be aware that such determinations, although useful for routine screening of possible allergies, are not sufficient to diagnose the disease. Clinical history and provocation tests should always be considered to confirm an allergy indicated by elevated allergen-specific IgE levels in serum. This specially applies to the diagnosis of mould allergy in general, and of Aspergillus fumigatusrelated allergic complications in particular.