The empty sella turcica is defined as a sella which, regardless of its size, is completely or partly filled with cerebrospinal fluid. An empty sella of normal size is a frequent and probably normal finding in unselected autopsy series. In clinical series an empty sella usually appears enlarged and is often associated with a variety of clinical disorders, constituting the so-called empty sella syndrome. Several causes of an enlarged empty sella have been suggested: a congenitally missing (deficient) sellar diaphragm with or without altered cerebrospinal fluid dynamics, previous pituitary gland hypertrophy or the outcome of a pituitary tumor necrosis. Increased intracranial pressure will induce a sellar enlargement in some patients and, consequently, also the emptiness. This pathogenesis is, however, applicable only in a minority of patients with an empty enlarged sella. Data from the literature and from own studies suggest that the enlarged empty sellae and the associated findings in the majority of cases are caused by spontaneous necrosis of a previous pituitary adenoma. This theory explains the frequent presence of pituitary insufficiency, pituitary hypersecretion, and visual field defects in patients with an empty sella. Furthermore, it offers an explanation of the finding of an empty enlarged sella in some patients with non-traumatic cerebrospinal fluid rhinorrhea and probably also benign intracranial hypertension. Thus, an empty enlarged sella is a stage in the spontaneous course of some pituitary adenomas and the associated findings constituting the empty sella syndrome are an occasional part of the clinical presentation of pituitary adenomas.