Forty-eight surgically removed pituitary adenomas have been investigated by electron microscopy. Distinct differences in fine structural appearances permitted their separation into 5 classes: 1) Growth hormone, (GH)-cell tumors; 2) Prolactin cell tumors; 3) Mixed adenomas composed of GH and prolactin cells; 4) Adrenocorticotrophic-Melanocyte stimulating hormone (ACTH-MSH) cell tumors; 5) Undifferentiated cell adenomas. Densely and sparsely granulated tumors were distinguished within classes 1, 2 and 4. Although these two forms appeared to represent well defined entities, they may be variants of the same tumor differing only in pace of hormone production and/or release. Number and size of secretory granules varied considerably among tumors composed of the same cell type indicating that pituitary tumor classification cannot be based solely on granule morphology. This classification takes into account morphogenesis with emphasis on clinical features and structure-function relationship.