On the bases of the authors analysed 64 cases of pituitary adenoma, endocrinologic manifestations and immunohistochemical staining by both conventional means and the antigen retrieval system. The incidence of each type and there interrelationship was thus demonstrated. The endocrinologic results of 64 cases were as follows: Should the total number of each type of adenoma be included here. Prolactin-secreting adenoma(39.0%), nonfunctioning adenoma(26.6%), and GH-secreting adenoma(17.2%). Conventional immunohistochemical staining revealed the presence of null cell adenoma(48.4%), prolactinoma(21.9%), and GH-secreting adenoma(12.5%). All adenomas shown by conventional immunohistochemical staining to be null cell type were thus reclassified by the antigen retrieval system. On the basis of these results, the authors concluded that for the classification of pituitary adenomas, immunohistochemical staining by the antigen retrieval system is more sensitive than conventional immunohistochemical staining. In addition to avoid false-positive results, EM confirmation is required.