Twenty-five adenomas of the thyroid were studied by transmission and scanning electron microscopy: 14 embryonal, fetal, and simple adenomas, 1 colloid adenoma, 2 toxic adenomas, 2 oxyphil adenomas, 2 "atypical adenomas" and 4 adenomas with capsular invasion, formerly designated "malignant adenomas." The only adenomas that presented with special ultrastructural features were the oxyphil and the toxic varieties. The toxic adenomas had organelle-rich cells with numerous and long microvilli on their surfaces. The oxyphil adenomas showed Hürthle cell changes, including abundant mitochondria, and smooth cell surfaces. The subdivision of the other forms of adenomas was impossible at the ultrastructural level; and this study did not reveal ultrastructural features which could distinguish the so-called "atypical" and "malignant" adenomas from ordinary thyroid adenomas. "Malignant" adenomas should preferable be called what they are: adenomas with capsular invasion. Neither does this study support the view that electron microscopy is a useful method to separate an adenoma from a well-differentiated follicular carcinoma in the occasional patient who presents problems.