The different biological properties of human IgG subclasses make each subclass unique in its functional role in either resistance to infection, autoimmune diseases or allergy. Not only are there marked differences in the relative concentrations of IgG subclasses in serum (IgG1> IgG2> IgG3/IgG4) but the distribution of the antibody responses in the 4 subclasses of IgG can vary markedly depending on the nature of the antigen, the type of infection, the degree of antigen exposure, the immunization regimens, the age of individual, the immune disorder and the allotype of the individual. Measurement of the IgG subclass distribution of antibodies can be informative in identifying an immunological deficiency, evaluating the production of host protective antibodies, and assessing pathophysiology. Determination of IgG subclass concentrations is essential in the diagnosis of immunodeficiencies. However, there is still uncertainty about the accuracy of measurements in relation to standards, monoclonal antibodies and assay types. For the paediatric population, a sensitive assay, such as an enzyme linked immunoabsorbent assay, is essential. A standardised definition of IgG subclass deficiency is yet to be accepted; however, values substantially below the 5th percentile for a normal healthy population of appropriate age measured by a defined assay system may be indicative of significant abnormality. There is emerging evidence that some subclass deficiencies are associated with increased susceptibility to infection. Such IgG subclass deficiencies may be amenable to treatment with intravenous gammaglobulin preparations, but further carefully designed and controlled studies are needed to ascertain treatment efficacy.