Sera of patients with Addison's disease contain autoantibodies recognizing antigen(s) in the adrenal cortex. In the present study we have examined the antigen expression in normal (n = 6) and pathological human adrenal tissues (n = 24) and also in the human steroid-producing adrenocortical cell line NCI-H295. Sera from two patients with Addison's disease were selected as they strongly stained the human adrenal gland and identified a 54 kDa autoantigen previously demonstrated as 21-hydroxylase. These sera reacted with normal human adrental cortex (n = 6), all hyperplasias (n = 5) and all the adrenocortical cancers (n = 9), whereas slight or no reactivity was observed in the adenomas without any detectable excess of peripheral steroids (n = 4). Both patient sera reacted in an identical manner with each tissue specimen and they also reacted strongly with the steroid-producing cell line. The data demonstrate that the expression of the Addisonian autoantigen correlates with the functional activity of adrenocortical neoplasms. Furthermore they suggest, that immunohistochemical stainings for steroid-producing enzymes may be clinically useful in the characterization of adrenal lesions.