Primary humoral immunodeficiency comprises a number of syndromes which in a descriptive manner indicate the nature and extent of the defect in the synthesis of specific antibodies and likewise of immunoglobulins. The understanding of humoral immunodeficiency has greatly advanced with the increase in knowledge about the cellular and molecular mechanisms of the development of B lymphocytes, which are the precursors of antibody-secreting plasma cells. This article reviews the advances made in the almost forty years that have passed since the first patient with agammaglobulinaemia was described. As far as X-linked agammaglobulinaemia is concerned, it is now clear that this is a disease of B lymphocytes, and that expression of the XLA gene prevents B cell development beyond the pre-B cell stage. Recent studies in patients with late-onset hypogammaglobulinaemia and selective IgA deficiency showed that there may be a common denominator for these two syndromes, since there is a close association with polymorphic antigens of the MHC class III region. Furthermore deletions or mutations of immunoglobulin genes can be the basis of selective deficiencies of one or several immunoglobulin isotypes. However, most of the humoral immunodeficiencies are based on defects in other non-immunoglobulin regulatory genes affecting or engaged in immunoglobulin-isotype synthesis. More recently patients have been described who have normal immunoglobulin isotype and complement levels and who show a selective defect in the antibody production to polysaccharide antigens. These patients most probably form a new disease entity in the spectrum of humoral immunodeficiency syndromes.