Abstract. A patient with Addison's disease, who was inadequately substituted with corticosteroids for twenty-five years, developed a pituitary tumour. The diagnosis of autoimmune thryroiditis with asymptomatic hypothyroidism was also made. ACTH and TSH plasma values were abnormally elevated. The ACTH secretion was suppressed by cortisone administered perorally. It seems possible that the pituitary enlargement is due to prolonged insufficient adrenocortical replacement therapy. The differential diagnosis and possible pathogenetic factors are discussed.