Addison's disease (chronic primary adrenal insufficiency) is commonly caused by autoimmune destruction of the adrenal cortex and is characterized by the presence of adrenal cortex autoantibodies in the serum. It can occur sporadically or in combination with other autoimmune endocrine diseases, that together comprise Type I or Type II autoimmune polyglandular syndrome. The microsomal autoantigen, steroid 21-hydroxylase enzyme has been shown to be the primary autoantigen associated with autoimmune Addison disease. Measurement of 21-hydroxylase autoantibodies aids in the diagnosis and management of patients of adrenal insufficiency.