Primary adrenal insufficiency (Addison's disease) often develops insidiously. Although a rare disorder, it is more common in type 1 diabetes mellitus. A 19 year old male with type 1 diabetes and autoimmune hypothyroidism experienced recurrent severe hypoglycaemia over several months, despite a reduction in insulin dose, culminating in an adrenal crisis. Recurrent severe hypoglycaemia resolved after identification and treatment of the adrenocortical insufficiency. In type 1 diabetes, undiagnosed Addison's disease can influence glycaemic control and induce severe hypoglycaemia.