A 16-year-old boy with insulin-dependent diabetes mellitus (IDDM) and a history of marginal glycemic control had severe hypoglycemia unawareness and a marked decrease in insulin requirement. His counterregulatory hormone response at the time of hypoglycemia suggested adrenocortical and adrenomedullary dysfunction. Further testing confirmed Addison disease. The patient's hypoglycemia unawareness was reversed by glucocorticoid replacement, although the plasma epinephrine response to hypoglycemia remained undetectable.