A broad classification of dysgammaglobulinemias characterized by low or undetectable serum levels of immunoglobulin class G (IgG). Deficiencies of IgG present variably according to subclass. IgG deficiencies are typically relative among subclasses and not absolute. Thus even with a given selective IgG subclass deficiency, total IgG levels may still fall within normal range. The clinical course and prognosis is dependent upon the severity of the deficiency and associated morbidity.