The clinical association between diabetes mellitus and auto-immune diseases of the thyroid, the adrenals and the gastric mucosa occurs more frequently than could be expected by chance. Sera from 133 patients with diabetes mellitus and 128 controls were therefore investigated for the presence of organ-specific auto-antibodies. Thyroid microsomal antibody and gastric-parietal-cell antibody were demonstrated with significantly increased frequency – 20% and 16% respectively – in sera from patients with diabetes mellitus. Antibodies reacting specifically with tissue components of the endocrine pancreas could not be demonstrated. From the data presented and from a review of the literature it is concluded that evidence is accumulating pointing to a disorder of the immunological system in patients with diabetes mellitus with regard to the formation of organ-specific humoral and cellular auto-immunity, and the occurrence of organ-specific auto-immune diseases.