The generally accepted classification of pituitary adenomas into eosinophilic, basophilic, mixed, and chromophobe types has not been very useful since no clear correlation exists between the staining character of the tumor cells and the clinical syndrome produced. The ultrastructural examination of different adenoma types with clinically manifest endocrine activity (acromegaly, amenorrhea-galactorrhea-syndrome, Cushing's disease) shows that the type of hormone secreted can only be determined in few cases. The ultrastructure is more representative of the activity of the secretory process than of the type of the product. Histoimmunological methods achieve the specific identification of the various types of adenomas with endocrine symptomatology. The so-called "chromophobe" adenomas, which only manifest signs of a space occupying lesion with pituitary insufficiency of varying degree and compression of the visual pathways, represent a mixed group. The majority of the cases (about 60%) shows increased prolactin secretion with amenorrhea or loss of potency, but without galactorrhea or gynecomastia. A small group of cases may either produce normal hormones (growth hormone or prolactin) at a very low rate, or secrete hormone fragments or abnormal, as yet undeterminable substances. The oncocytomas, which stain with eosin in light microscopy, seem to suffer from a defective metabolism and therefore may have lost the ability of hormone production.