Adrenal cortex antibodies (ACA) were measured by immunofluorescence in 8840 adult patients with organ-specific autoimmune diseases without overt hypoadrenalism. Sixty-seven (0.8%) patients were ACA-positive, with the highest prevalence in those with premature ovarian failure (8.9%). Forty-eight ACA-positive and 20 ACA-negative individuals were enrolled into a prospective study. Antibodies to steroid 21-hydroxylases (21-OH), steroid 17 alpha-hydroxylase (17 alpha-OH) and cytochrome P450 side chain cleavage enzyme (P450scc) were measured by immunoprecipitation assay. Human leucocyte antigens D-related (HLA-DR) genotyping was also carried out and adrenal function assessed by ACTH test. On enrollment, 75% of ACA-positive patients had a normal adrenal function, while 25% revealed a subclinical hypoadrenalism. 21-OH antibodies were positive in 91% of ACA-positive sera. Eleven patients were positive for steroid-cell antibodies by immunofluorescence, and 9 revealed a positivity for antibodies to 17 alpha-OH and/or P450scc. During the prospective study, overt Addison's disease developed in 21% and subclinical hypoadrenalism in 29% of ACA-positive patients, while 50% maintained normal adrenal function. Progression to Addison's disease was more frequent in patients with subclinical hypoadrenalism, high titers of ACA and higher levels of 21-OH antibodies, complement-fixing ACA and HLA-DR3 status. All 20 persistently ACA-negative patients were also negative for antibodies to 21-OH, 17 alpha-OH, and P450scc, and all maintained normal adrenal function during follow-up. In conclusion, the detection of ACA/21-OH antibodies in adults is a marker of low progression toward clinical Addison's disease.