This review summarizes current knowledge on pathology of proliferative lesions of the human pituitary. The morphologic classification of pituitary adenomas--based on histology, immunohistochemistry and electron microscopy--has now been firmly established. It has been conclusively proven that all presently recognized adenohypophysial cell types give rise to adenoma and all known pituitary hormones may be secreted in excess. Evidence is accumulating that hyperplasia of various adenohypophysial cell types can lead to hypersecretory syndromes similar to those associated with the corresponding adenomas. Owing to the rarity of studies on pituitary hyperplasia, the condition is still incompletely defined. The difficulties regarding morphologic diagnosis of pituitary hyperplasia are discussed. Despite major advances in the field of pituitary pathology, several problems concerning structure-function relationship, as well as pathogenesis of proliferative lesions are still unresolved. There is strong circumstantial evidence suggesting that the cytological mapping of the pituitary is incomplete and there are still cell types waiting to be discovered.