This study aimed to investigate the prevalence of IgG subclass deficiency in wheezy children aged <3 y. Serum levels of IgA, IgE and IgG subclasses were measured in 310 children with recurrent wheezing and in 100 healthy controls. IgG3 levels were below the normal lower limit in 123 (39.6%) patients. This finding may reflect delayed maturation of the immune system, predisposing young children <3 y of age to wheezing.