Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disease caused by ACTH resistance and leads to isolated glucocorticoid deficiency. Although FGD patients typically have normal mineralocorticoid secretion, subtle alterations in the renin-angiotensin-aldosterone axis have been reported in a subset of patients at presentation. Anecdotally, some patients with FGD have been initially diagnosed as having Addison's disease (AD), with implications for treatment and genetic counselling. Currently, mutations in three genes: the ACTH receptor (MC2R); the melanocortin 2 receptor accessory protein (MRAP); and the steroidogenic acute regulatory protein (STAR) are known to give rise to FGD types 1-3. We investigated a cohort of autoantibody-negative AD patients for mutations in these genes.