Intranuclear pseudoinclusions are well known in papillary carcinomas of the thyroid gland, hepatocellular carcinomas, meningiomas, paragangliomas, pheochromocytomas, and melanomas. Only two papers on the intranuclear inclusions of adenohypophyseal cells in humans have been reported. This study found that intranuclear cytoplasmic pseudoinclusions occur frequently in pituitary adenoma cases (70.3%, 97 of 138 pituitary adenomas) and are uncommon in normal pituitary tissue (11.1%, 1 of 9 normal pituitary tissues). In addition, the frequency of intranuclear cytoplasmic pseudoinclusions between the functional and non-functional pituitary adenomas was found to be similar. Electron microscopy and immunostaining was used to reveal the entity of the intranuclear inclusion. These intranuclear inclusions are due to cytoplasmic invagination because 1) the inclusions are continuous with the cytoplasm, 2) all cytoplasmic organelles, such as the endoplasmic reticulum, the Golgi apparatus, and the secretory granules are found in the inclusions, 3) immunoreactivity of the intranuclear inclusion is the same as that of the cytoplasm. In conclusion, intranuclear cytoplasmic pseudoinclusions in pituitary adenomas occur frequently (70.3%) and are formed by cytoplasmic invagination. This study suggests that pituitary intranuclear inclusions caused by cytoplasmic invagination be called "intranuclear cytoplasmic pseudoinclusions".