Immunoglobulin A deficiency is the commonest immunodeficiency syndrome, occurring in 1 in 600 patients. It is especially common in Caucasians. The most well-defined aspect of the syndrome is the development of severe allergic reactions to the IgA contained in transfused blood. Patients probably have an increased incidence of sinopulmonary infections and chronic diarrheal illness, although the increased susceptibility may be attributed to concomitant IgG subclass (especially IgG2 and IgG4) deficiency. There is no effective treatment for the IgA deficiency although those with IgG subclass deficiency and recurrent bacterial infections may benefit from immunoglobulin infusions.