The frequency of polyglandular autoimmune syndrome Type II, (idiopathic or autoimmune Addison's disease associated with autoimmune thyroid disease, and/or insulin-dependent diabetes mellitus), was retrospectively investigated in 44 patients (26 females and 18 males) with idiopathic Addison's disease (median age at onset 32.5 years, range 8-62; median observation time 17 years, range 0.5-41) evaluated between 1966 and 1988 in the Department of Endocrinology, General Hospital, Malmö. Twenty-two patients (16 females and 6 males) fulfilled the criteria for polyglandular autoimmune syndrome Type II and of these 16 had autoimmune thyroid disease and 9 insulin-dependent diabetes mellitus. In 7 of 9 patients insulin-dependent diabetes mellitus developed prior to Addison's disease, whereas autoimmune thyroid disease was diagnosed before as well as after Addison's disease and with a similar age distribution as Addison's disease. In conclusion, the present study indicates that polyglandular autoimmune syndrome Type II occurs in about 50% of patients with idiopathic Addison's disease and that autoimmune thyroid disease is more common than insulin-dependent diabetes mellitus in these patients. The present study also indicates that insulin-dependent diabetes mellitus precedes Addison's disease in a majority of these patients and usually develops at a young age, whereas autoimmune thyroid disease precedes as well as succeeds Addison's disease and occurs at all ages in these patients.