Electron microscopy of an amphophil pituitary adenoma surgically removed from a 51-year-old woman who had Nelson's syndrome revealed that the tumor was composed of melanocorticotroph cells. This finding is consistent with the view that in the human pituitary gland one single cell type produces both adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH). In contrast to the ultrastructure of pituitary adenomas associated with Cushing's syndrome, no or only very few microfilaments were detected in the cytoplasm of the tumor cells, suggesting that adrenocortical steroids are required for the formation of microfilaments. The presence or absence of microfilaments in the tumor cells may be regarded as a distinguishing ultrastructural feature between Cushing's syndrome and Nelson's syndrome. It appears that changes in the level of circulating corticoids may affect the ultrastructural features of melanocorticotroph cells not only in normal pituitaries but also in adenohypophyseal adenomas.