&NA;We present a case of Nelson's syndrome in which the clinical, radiographic, and surgical findings are indicative of a discrete microadenoma rather than of diffuse pituitary hyperplasia. Ultrastructural analysis reveals this to be the first example of a sparsely granulated lesion in Nelson's syndrome and only the second such adrenocorticotropic hormonesecreting tumor reported. Unfortunately, there do not seem to be any firm ultrastructural features that differentiate between hyperplasia and discrete adenoma. Separation of the two entities may be important in delineating the pathogenesis and treatment of Nelson's syndrome; these are discussed.