Studying 239 Type 1 (insulin-dependent) diabetic patients and 144 of their first-degree relatives, we found a significant prevalence of autoimmune manifestations in both groups, compared with sex- and age-matched control subjects (p less than 0.001). In particular, in diabetic patients we found a high frequency of autoimmune thyroid disease and idiopathic Addison's disease and also a significant prevalence of thyroid (p less than 0.001), parietal cell (p less than 0.05) and adrenal antibodies (p less than 0.05). In the relatives a high frequency of thyroid disease, thyroid, parietal cell and adrenal antibodies and a significant prevalence of islet cell antibodies (p less than 0.05) were detected. In both groups functional glandular tests and gastric biopsies performed on the basis of autoantibody positivity revealed 13 examples of subclinical hypothyroidism, two cases of reduced adrenocortical reserve and five of atrophic gastritis. Autoantibody screening in diabetic patients and their relatives permitted the early diagnosis of the underlying endocrine disorders.