Previous studies conclusively demonstrated transformation of somatotrophs into bihormonal mammosomatotrophs in gestational lactotroph hyperplasia during pregnancy. Similar transdifferentiation of somatotrophs into thyrotrophs through bihormonal intermediate thryrosomatotrophs was documented during thyrotroph hyperplasia in both rodent and human pituitaries in hypothyroidism. The cessation of the stimulation resulted in reversal of the process in both conditions. The conversion of lactotrophs into somatotrophs was suggested but not documented previously in the human gland. The present study was undertaken to investigate cases of somatotroph hyperplasia by transmission electron microscopy, immunoelectron microscopy using double immunogold labeling for growth hormone and prolactin, as well as combined immunocytochemistry and in situ hybridization. Adenohypophysial tissue was removed from a 38-year-old man and a 29-year-old woman with longstanding acromegaly due to ectopic overproduction of growth hormone–releasing hormone (GRH) by bronchial carcinoid tumors. For comparison, two pituitary biopsies were studied: one from a 38-year old woman with idiopathic lactotroph hyperplasia and one from a 14-year-old boy with secondary lactotroph hyperplasia due to a suprasellar craniopharyngioma. In the patients with somatotroph hyperplasia, the prevailing cell type was the hyperplastic somatotroph joined by mammosomatotroph deriving from lactotrophs, whereas monohormonal lactotrophs were rare. The predominance of mammosomatotrophs and active lactotrophs was documented in the patient with idiopathic lactotroph hyperplasia, whereas the case of the patient with secondary lactotroph hyperplasia was characterized by monohormonal lactotrophs and somatotrophs, but mammosomatotrophs were rare. That finding in the pituitary of the boy suggests that participation of mammosomatotrophs in lactotroph hyperplasia is not unconditional. Our findings conclusively demonstrate conversion of lactotrophs into mammosomatotrophs during somatotroph hyperplasia, providing further evidence for the potential of reversible transdifferentiation between somatotrophs and lactotrophs in response to functional demand.