5,100 formalin-fixed pituitaries of an unselected autopsy material of the Institute of Pathology of the Ernst-Moritz-Arndt-University of Greifswald were analysed with regard to the occurrence of adenomas and adenomatous hyperplasias. Beside the histological classification the size was determined. For some adenomas of the pituitary gland (ACTH-cell-adenomas, TSH-cell-adenomas and evidently also prolactinomas) the development from a hyperplasia which is a consequence of a central regulation disorder is supposed. The pathogenesis of the other types of adenomas remains still unclear. Difficulties in differential diagnosis between adenomatous hyperplasias and small adenomas are discussed.