This review summarizes current knowledge on pituitary changes in patients with acromegaly. The histologic, immunohistochemical and electron microscopic study provided conclusive evidence that a marked diversity exists between the tumors which secrete growth hormone (GH) in excess, such as densely and sparsely granulated GH cell adenoma, the mixed GH prolactin cell adenoma and the mammosomatotrope adenoma. The latter two tumors produce GH and prolactin simultaneously. Densely granulated GH cell tumors may produce thyrotropin and alpha subunit as well. Somatotrope carcinomas are extremely rare. GH cell hyperplasia can also be associated with acromegaly in patients with extrapituitary GH-releasing hormone secreting tumors. The medical therapy of acromegaly is reviewed briefly, including long-acting somatostatin analogs and pegvisomant, a GH receptor blocker.