A 52 year-old male with a history of Cushing's Disease at age 18 and bilateral adrenalectomy at age 23 presented with visual changes. An MRI scan showed a pituitary macroadenoma (Nelson's syndrome). Other than the development of diabetes mellitus at age 32, his disease was stable until presentation. Serum studies show markedly elevated ACTH levels, but he had no hyperpigmentation. The tumor was removed by endoscopic surgery. Microscopic examination showed a pituitary adenoma with strong immunostaining for ACTH. In addition, the tumor cells showed Crooke's hyaline change and stained strongly for cytokeratin (Crooke's Cell Adenoma). Normal pituitary was not present. Crooke's cell adenomas are extremely rare and have not been previously reported in Nelson's Syndrome.