Some 5% to 10% of all infants and toddlers suffer from four or more episodes of otitis per year. Usually, this is a temporary problem that resolves with increasing age. In a minority of cases, otitis episodes are frequent or have an abnormal course, with complications such as mastoiditis. In these cases, immunologic screening is indicated, to exclude an immunodeficiency. Agammaglobulinemia or hypogammaglobulinemia is rare among these patients. Other immune defects that occur more often are deficient or lowered immunoglobulin (Ig)A or decreased levels of one or more IgG subclass, in particular IgG2. The specific antibody response to bacterial capsular polysaccharides often is disturbed. These findings can give direction to the treatment of children with frequent, recurrent otitis.