Nelson's syndrome is usually characterized by hyperpigmentation, elevated level of ACTH, and pituitary adenoma, which is frequently available as macroadenoma. It is usually occurred in the patients suffering from Cushing's syndrome. Growth of tumor has been revealed in 10-30% of the patients following adrenalectomy. Tumors are often benign in Nelson's syndrome. The selected treatment of Nelson's syndrome are transsphenoidal or transfrontal microsurgery. The role of radiation as prophylactic effect in the patients following adrenalectomy is unknown. In this research Nelson's syndrome have been studied on the patients records who have been hospitalized in Dr.Shariati hospital during the last 10 years. The achieved results show that, among the 49 patients suffering from Cushing syndrome who have referred after operation, 11 patients (22.4%) of them have been affected with Nelson's syndrome. The syndromes have been appeared after 3-9 months and with regard to occurrence of Nelson's syndrome in some patients immediately after bilateral adrenalectomy, it is recommended that selecting of the patients for bilateral adrenalectomy should be studied accurately