This is the first report of Cushing's syndrome under oral budesonide treatment. An 81-year-old man known for paroxysmal atrial fibrillation and chronic renal insufficiency, treated with 6 mg budesonide for collagenous colitis, developed Cushing's syndrome under co-administration of amiodarone. The Cushing's syndrome disappeared after discontinuation of the amiodarone treatment. Metabolism of the two medications by hepatic cytochrome P 450 3A may explain the development of Cushing's syndrome.