Sera from 300 Italian patients with Addison's disease were collected over a 30 year period. Among these patients, 82% had autoimmune disease, 13% had tuberculosis and 5% had another causal condition. In 59% of the cases, autoimmune disease was associated with the autoimmune manifestations contributing to the description of polyglandular autoimmune disease (PGAD). In PGAD type 1, the disease was associated with chronic candidiasis and/or chronic hypoparathyroidism. In PGAD type 2, the patients had autoimmune thyroid disease and/or diabetes mellitus type 1, and in PGAD type 4, they presented a combination with other autoimmune diseases excluding those previously mentioned. Finally, the autoimmune disease was apparently isolated in 41% of the cases. In addition, patients with these four forms of disease exhibited a different genetic pattern, sex distribution, and age at presentation in addition to minor frequency of autoimmune diseases. Adrenal cortex autoantibodies directed against 21-hydroxylase were common serological markers for these four main clinical forms, showing a very high frequency at clinical onset of adrenal insufficiency. In some patients, steroid-producing cell autoantibodies were also present and correlated with gonadal failure and they recognize of 17alpha-hydroxylase or P450 side chain cleavage enzymes as target antigens.