Comprehensive article summarizing more than 25 years of experience with pituitary hyperplasia in surgical material. Morphologic forms of hyperplasia<197>diffuse and nodular<197>are defined and, for comparison, the normal morphology, frequency and intraglandular distribution of cell types are briefly reviewed. All cell types can give rise to hyperplasia, although their frequency, extent and clinical importance widely vary. Somatotroph hyperplasia is rare; it is limited to cases of GHRH overproduction by extrapituitary endocrine neoplasms and sporadic examples of gigantism. Prolactin cells display the highest propensity for non-neoplastic proliferation. Physiologic hyperplasia occurs in pregnancy and lactation. Pathological hyperplasia is mostly secondary to other, neoplastic or non-neoplastic, space occupying processes. Idiopathic lactotroph hyperplasia is very rare. The much-disputed corticotroph hyperplasia is infrequent cause of pituitary dependent Cushing's disease. Despite difficulties of diagnosis in fragmented biopsies, several well-documented cases prove the existence of corticotroph hyperplasia which is nearly always nodular. Thyrotroph hyperplasia, secondary to hypothyroidism, a treatable condition, is not expected to occur in surgical material, yet several cases have been identified. Operated lesions are massive nodular leading to significant pituitary enlargement thereby mimicking TSH- or PRL-producing adenoma. Hyperprolactinemia is a frequent concomitant of severe thyrotroph hyperplasia. Gonadotroph hyperplasia and proliferation of pars intermedia derived POMC cells are not likely to occur in surgical material and have no clinical significance. Adenoma formation may rarely be associated with any type of pituitary hyperplasia.