Selective IgG subclass (IgGSc) deficiencies are frequently found in association with recurrent infections in childhood. IgG1 deficiency is the most severe and is associated with features typical of panhypogammaglobulinaemia. Immunoglobulin replacement therapy is usually required. IgG2 deficiency is associated with recurrent infections with encapsulated bacteria such as Haemophilus influenzae and Streptococcus pneumoniae. IgG2 deficiency may be transient in children under five years of age and patients improve with antibiotics and immunisation. IgG3 and IgG4 deficiency are commonly found in children with recurrent infections and may indicate a disordered immune system since absence of these antibodies alone appears insufficient to cause symptoms. Children may also have selective IgGSc deficiencies in the absence of recurrent infections. This is explained by compensatory factors in other parts of the immune system. Measurement of IgGSc levels should be based on highly specific polyclonal antisera which show no IgGSc cross-reactivity. Most monoclonal antibodies are unsatisfactory since allotypes are detected variably, leading to excess reporting of IgGSc deficiencies and Mabs cannot be used for nephelometric or turbidimetric methods.