Increased levels of serum IgD can be found in single patients with a variety of clinical syndromes and in the disease entity designated hyper-IgD syndrome which is associated with periodic fever and lymphadenopathy. We investigated 17 patients, both children and adults, with high serum IgD levels ranging from 220 to 5300 IU/ml. Eight patients had periodic fever and lymphadenopathy, four showed a humoral immunodeficiency, and the remainder had a variety of clinical abnormalities. Serum IgA levels were consistently high in all patients except in those with an immunodeficiency. Serum IgD complexes were detectable in each serum, which indicates that the occurrence is not pathognomic for the syndrome of periodic fever. Antibody formation against the primary antigen Helix pomatia hemocyanine and the secondary antigen tetanus toxoid showed no abnormalities in the patients without an immunodeficiency. Bone marrow origin of serum IgD was strongly suggested by enumeration of IgD-containing plasma cells. We conclude that no apparent relationship exists between the several clinical syndromes and increased serum IgD.