Two HLA-identical sisters have developed the full picture of type I polyglandular autoimmune syndrome over a period of 12 years. Both girls have hypoparathyroidism and Addison's disease. One of them additionally developed diabetes mellitus, hypergonadotropic hypogonadism and hypothyroidism. Autoantibodies to the adrenal, parathyroid and thyroid glands are present in both patients, as well as antinuclear antibodies. HLA associations have been described recently for the type I polyglandular autoimmune syndrome, but this could not be confirmed in the present two cases. Although we assume that the same genetic defect is present in both girls, additional factors to the genetic disposition are important for the clinical expression of the disease. The linkage of the disease-causing gene with the HLA region is not very close.