The ten subjects of this report who had concomitant idiopathic, adult-onset immunoglobulin (Ig) deficiency and pernicious anemia, presented a relatively uniform syndrome. Severe impairment of humoral antibody production was associated with a marked propensity for bacterial infections which appeared in most instances before the pernicious anemia was detected. Atrophic gastritis, achlorhydria and absent intrinsic factor were associated in most instances with the characteristic findings of pernicious anemia. Unusual features, however, included the strikingly early onset of pernicious anemia (mean age thirty-four years), the absence of antibodies to parietal cells and intrinsic factor, frequent diarrhea and giardiasis, and malabsorption of vitamin B12 in the presence of intrinsic factor. One patient had ulcerative colitis, another rheumatoid-like arthritis and a third was allergic to a number of allergens. The absence of serum antibodies in these patients provides the strongest evidence to date that serum antibodies are not essential in the pathogenesis of the atrophic gastritis of pernicious anemia or of other autoimmune disorders. The earlier clinical appearance of Ig deficiency than of pernicious anemia and the frequent occurrence of a variety of other autoimmune diseases in hypogammaglobulinemia, suggest that the Ig deficiency may be the primary lesion. Since delayed cell-mediated immune mechanisms appear to be intact in these patients, it seems possible that the gastric atrophy resulted from delayed type cellular immune hyperactivity, host hypersensitivity or both. © 1969.