The serum immunoglobulin E concentration was determined in groups of patients with immunologic deficiency diseases that were secondary to a variety of pathophysiologic mechanisms including decreased synthesis, accelerated catabolism and excessive loss of immunoglobulins. The geometric mean IgE level of normal adults was 105 ng/ml with a 95% interval of 5 to 2045. All of the patients with a generalized reduction in immunoglobulin production had a markedly reduced level of IgE. In nine of 10 patients with idiopathic-acquired hypogammaglobulinemia the IgE level was undetectable (i.e., < 4 ng/ml) and was only 5 ng/ml in the remaining patient. Similarly, IgE was undetectable in the serum of seven of eight patients with thymoma and hypogammaglobulinemia and was only 8 ng/ml in the remaining patient. The IgE levels of groups of patients with hypogammaglobulinemia secondary to a shortened immunoglobulin survival were found to be normal in contrast to the markedly reduced IgE levels seen in the patients with a generalized defect in immunoglobulin synthesis. The geometric means of the serum IgE concentrations of the 10 patients with myotonic dystrophy, low IgG concentration and short IgG survival, the two patients with familial hypercatabolic hypoproteinemia and a short survival of many serum proteins, and the 10 patients who had intestinal lymphangiectasia and generalized loss of serum proteins into the gastrointestinal tract did not differ significantly from normal. The 12 patients with Wiskott-Aldrich syndrome had markedly elevated IgE levels with a geometric mean value of 3475 ng/ml. These high IgE levels may reflect chronic stimulation of the IgE synthesizing system by antigens that cannot be processed adequately and eliminated by these patients.