The microcysts found in two adenohypophyses removed from patients with nonsecreting adenomas of this gland have been studied. In one case the cysts were completely surrounded by neoplastic tissue, whereas in the second they were located within the normal parenchyma. The cysts were surrounded by a basement membrane, and appeared to contain a finely granular weakly electron-dense substance. Their walls were lined by mostly cubic cells with microvilli, joined by junctional complexes. Intracytoplasmic microlumina could be seen in some of these cells. Cells showing cilia with a 9 + 2 fibrillar pattern and cells with mucosecretory differentiation were also present. Furthermore endocrine cells were also found in the cyst-lining epithelium. The ultrastructural aspects of the cyst cells lead to reject the hypothesis that these are derived from folliculostellate cells. Besides, intermediate aspects between cyst cells and tumor cells were never visible. Therefore it seems likely that a relationship between adenoma and cyst cells does not exist and that cyst cells originate from remnants of Rathke's pouch with a ciliate or mucosecretory differentiation. The presence of endocrine cells in the cyst-lining epithelium deserves further study, for it seems to indicate that an endocrine differentiation may occur in the adult, too. To our knowledge this is the first ultrastructural study of these structures in adult human pituitary gland.