Isolated ACTH deficiency is a rare disorder characterized by the hypocortisolism which is due to the defect of ACTH synthesis or to the disturbed release of ACTH, with other pituitary hormone being normal. The following is the one of the cases.A male patient of 27 years of age, complaining progressive weight loss, showed no cortisol response to the test of rapid and long ACTH stimulation, with low basal serum cortisol, ACTH, and 24 hour urine free cortisol. No ACTH response was made to the vasopressin, insulin-induced hypoglycemia and metyrapone test. But he responsed normally to the combined pituitary stimulation test except to the tests of ACTH and growth hormone. No abnormality was found in sella and adrenal CT. Through thyroid evaluation, primary hypothyroidisim was observed with positive antimicrosomal and antithyroglobulin antibody. The needle aspiration biopsy of thyroid showed lymphocytic infiltration which was compatible with Hashimoto's thyroiditis.Here we present the isolated ACTH deficiency combined with autoimmune lymphocytic thyroiditis, which cause is suspected to autoimmune. (J Kor Soc Endocrinol 5:206~212, 1990)