Unexplained hypotension during a stay in the ICU is not uncommon in patients who underwent major surgery such as open heart surgery. When the cardiac output of patients is low, the heart may be the origin of this problem. If the heart function is normal, then we have to consider adrenal insufficiency as a possible cause of this hypotension. Adrenal insufficiency is a rare condition in the general population, yet patients who are under a stressful condition might experience adrenal insufficiency more frequently. We report here on a case of a patient who was in an unstable postoperative state with adrenal insufficiency after surgery and the patient dramatically recovered after the replacement of hydrocortisone.