Hyper-IgE syndrome (HIES) is a primary immunodeficiency characterized by atopic manifestations and susceptibility to infections with extracellular bacteria and fungi, which frequently occur in the skin and lung. Atopic manifestations in HIES include extremely high serum IgE levels, eczema and eosinophilia. Most of the extracellular bacterial infections are associated with disproportionally milder inflammation than normal, which was originally described as having a 'cold abscess'. Non-immunological abnormalities are also observed in most patients with HIES, including a distinctive facial appearance, scoliosis, hyper-extensive joints and retained primary teeth. Recent studies have demonstrated that hypomorphic mutations in signal transducer and activator of transcription 3 result in the classical multisystem form of HIES, whereas a null mutation in tyrosine kinase 2 causes the autosomal recessive form of HIES that is associated with viral and mycobacterial infections. Analyses of cytokine responses in both types of HIES have revealed defects in signal transduction for multiple cytokines including IL-6 and IL-23, leading to impaired T(h)17 function. These results suggest that the defect in multiple cytokine signals is the molecular basis of the immunological and non-immunological abnormalities in HIES and that the susceptibility to infections with extracellular bacteria and fungi in HIES might be associated with the defect in T(h)17 cell differentiation.