Humoral (or antibody) immunodeficiency syndromes may occur as apparent congenital or acquired abnormalities, with deficiencies in all or in only some classes of immunoglobulins. Most patients are recognized because of recurrent infections with high-grade extracellular encapsulated bacterial pathogens, but some with selective IgA deficiency or with transient hypogammaglobulinemia of infancy may have few or no infections. Although general population statistics are not available, most defects are thought to be rare; humoral immunodeficiency is more prevalent than cellular immunodeficiency, possibly due to early death from the latter defects. Disorders affecting B-cell function may be inherited as X-linked recessive or as autosomal traits. Although considerable information exists about such defects at a functional and cellular level, the primary biologic errors are as yet unknown for all of them. Apparent abnormalities of B-cell maturation and/or intrinsic B-cell malfunction are seen in a majority of these defects. The heterogeneity of B-cell morphology and function in large pedigrees of patients with X-linked agammaglobulinemia makes it unlikely that the defect is due to a distinct gene rearrangement abnormality at a specific stage of B-cell maturation. Early recognition of B-cell deficiency and institution of adequate immunoglobulin replacement therapy can prevent extensive damage to the lungs and other life-threatening problems from infection and allow a relatively normal childhood and adult life.