A 28 year old woman presented with symptoms and biochemical findings suggesting hypoglycemia. Detailed endocrine investigations indicated secondary adrenocortical insufficiency with no rise in plasma ACTH or plasma cortisol following insulin-induced hypoglycemia or vasopressin infusion. The adrenal glands however produced cortisol normally following prolonged stimulation with depot tetracosactrin. Other anterior pituitary hormones were released normally following appropriate stimulation tests. The literature on isolated ACTH deficiency is briefly reviewed, and this case represents one of the few reports in which the diagnosis has been substantiated by ACTH radioimmunoassays.