This case study describes a 33-year-old man with Addison's disease who presented with increased plasma adrenocorticotropic hormone (ACTH), normal plasma cortisol, and absent diurnal rhythms. Magnetic resonance imaging (MRI) indicated pituitary hyperplasia. Conventional hydrocortisone replacement therapy may not inhibit high ACTH levels in the morning; however, replacing hydrocortisone with dexamethasone achieved good therapeutic results.