Primary adrenal insufficiency is manifested by a deficiency of adrenal cortex hormones and can lead to a life-threatening condition. Early diagnosis is key to patient survival. Auto-antibodies to one of the adrenal steroidogenesis enzymes, 21-hydroxylase, are an immunological marker of autoimmune adrenal insufficiency. On the one hand, the study of antibodies to 21-hydroxylase is a method that helps establish the etiology of the disease – the autoimmune genesis of adrenal gland damage. On the other hand, the determination of autoantibodies to 21-hydroxylase is the only prognostic factor of the risk of adrenal insufficiency, which makes it possible to prevent the development of acute adrenal crisis. The article provides a brief literature review on autoantibodies to 21-hydroxylase and the pathogenesis of autoimmune adrenal insufficiency, and a series of clinical cases that illustrates the significant role of autoantibodies to 21-hydroxylase in diagnosis of adrenal insufficiency.