Objective: To evaluate the frequency of autoantibodies (Ab) against 21 hydroxylase (21OH), side-chain cleavage (SCC) and 17a-hydroxylase (17OH), in Addison's disease (AD) and autoimmune polyendocrine syndrome type III (APSIII). Design and Methods: We used radiobinding assays and in vitro translated recombinant human 35 S-21OH, 35 S-SCC or 35 S-17OH and studied serum samples from 29 AD (18 idiopathic, 11 granulomatous) and 18 APSIII (autoimmune thyroid disease plus type 1 diabetes mellitus, without AD) patients. Results were compared with those of adrenocortical autoantibodies obtained with indirect immunofluorescence (ACA-IIF). Results: ACA-IIF were detected in 15/18 (83%) idiopathic and in 1/11 (9%) granulomatous AD subjects. 21OHAb were found in 14/18 (78%) idiopathic and in the same (9%) granulomatous AD subject. A significant positive correlation was shown between ACA-IIF and 21OHAb levels (r 2 ¼ 0.56, P < 0.02). The concordance rate between the two assays was 83% (24/29) in AD patients. SCCAb were found in 5/18 (28%) idiopathic (4 of whom were also positive for 21OHAb) and in the same (9%) granulomatous AD subject. 17OHAb were found in only 2/18 (11%) idiopathic and none of the granulomatous AD patients. Two APSIII patients were positive for ACA-IIF, but only one was positive for 21OHAb and SCCAb. 17OHAb were found in another two APSIII patients. Conclusions: Measurement of 21OHAb should be the first step in immune assessment of patients with AD and individuals at risk for adrenal autoimmunity, in addition to ACA-IIF. Due to their low prevalence in AD, measurement of SCCAb and 17OHAb should be indicated only for 21OHAb negative patients and/or for those with premature ovarian failure, regardless of ACA-IIF results.