Isolated adrenocorticotropin (ACTH) deficiency is a rare cause of secondary adrenocortical insufficiency. This review summarizes the clinical and laboratory features of 39 previously reported cases plus 4 new patients. The clinical manifestations of isolated ACTH deficiency are variable, nonspecific and similar to those seen in adrenocortical insufficiency of any cause. The diagnosis of isolated ACTH deficiency due to intrinsic pituitary disease is made unequivocally when all the following criteria are met: 1) low basal urinary 17-hydroxycorticosteroid (17-OHCS) levels with or without low basal plasma cortisol, 2) low or normal basal plasma ACTH, 3) stimulation of cortisol, 17-OHCS or both during prolonged ACTH administration, 4) lack of 17-OHCS elevation in response to metyrapone and 5) normal secretory indices of other pituitary hormones. Isolated ACTH deficiency secondary to suprapituitary (e.g., hypothalamic) dysfunction is also based upon the above criteria, but, in addition, is associated with stimulation of cortisol and ACTH secretion following vasopressin administration.