Clinically non-functioning pituitary adenomas are morphologically classified into two groups, those which have hormone immunoreactivity and ultrastructural features of known adenohypophysial cell types but are clinically silent, and those composed of cells that do not resemble nontumorous adenohypophysial cell types. Among the former are the silent somatotroph adenomas, silent corticotroph adenomas and silent gonadotroph adenomas; the latter include the silent type III adenomas, null cell adenomas and oncocytomas. We review their histological, immunohistochemical and ultrastructural features, the results of in situ hybridization to determine hormone synthesis by these tumors and data obtained from tissue culture characterizing their hormone release in vitro. Non-functioning adenomas represent a heterogeneous group. The discrepancies between morphology, immunoreactivity and lack of endocrine activity of silent adenomas are not clear. Oncocytomas are variants of null cell adenomas. We suggest that null cell adenomas and oncocytomas originate in uncommitted pluripotent precursor cells capable of undergoing multidirectional differentiation. The progenitor cells differentiate most frequently toward FSH/alpha-subunit producing cells; the mechanism of preferential differentiation is obscure.