The usefulness and limitations of electron microscopy (EM) in pituitary tumor diagnosis are reviewed and illustrated with clinical examples. The traditional classification of chromophil and chromophobe adenomas is often inconsistent with the hormonal activity of the tumors. Virtually all pituitary adenomas contain some secretory granules when viewed with EM. Endocrine inactive chromophobe adenomas contain 150 nm granules with no demonstrable hormone function. Typical growth hormone (GH) secreting eosinophil adenomas contain large 375 nm granules which dominate the cell cytoplasm. GH secreting chromophobic tumors contain secretory granules of abnormal size and concentration which are invisible to the light microscopist. The variability in granule size may indicate the production of abnormal granules or reflect the stage of the cell in a secretory cycle. Because of this wide range in granule size, the identification of tumor cell type or hormone produced is not reliable by granule measurement alone. Some neoplasms in the sella turcica may be so bizzare or undifferentiated as to defy classification. In such instances, EM can reveal ultrastructural details which identify their origin from pituitary tissue. Malignant pituitary tumors may contain minute secretory granules, and rare pituitary oncocytomas are packed with abnormal mitochondria.