Isolated ACTH deficiency is a uncommon disorder causing secondary adrenocortical insufficiency. Less than 200 cases have been reported in the world. The major clinical manifestations are hypoglycemia, weight loss, hypotension, anemia, weakness, nausea, inability to excrete water load, and hyponatremia. A 56-year-old male was admitted because of weakness, fatigue, nausea and vomiting. He was pallor and not associated with hyperpigmentation. The basal plasma ACTH and cortisol levels were 8.30 pg/ml and 0.6 ug/dl. The serum cortisol did not response to rapid ACTH stimulation test. On combined pituitary stimulation test, the cortisol did not response to insulin-induced hypoglycemia. Other anterior pituitary hormones showed normal responses except elevated prolactin level. Six months after glucocorticoid replacement therapy, the elevated basal prolactin level returned to normal. Brain MRI did not show any anatomic abnormalities of the sellar and suprasella area. We report a case of isolated ACTH deficiency accompanied by hyperprolactineia, which respond to glucocorticoid replacement therapy.