Serum IgD levels were determined in 66 patients with well-defined primary immunodeficiency diseases. The two major groups of patients consisted of those with ataxia-telangiectasia (38 patients) and those with selective IgA deficiency (11 patients). The ataxia-telangiectasia patients tended to have higher serum IgD levels while no significant difference was found in the serum IgD levels of the selective IgA deficiency patients when compared with the controls. No correlation was found between the IgD levels and the presence of frequent infections in both patient groups or the associated disorders present in the selective IgA deficiency patients. The percentage of low serum IgD phenotype in the normal subjects was similar to that described in the literature.