Five of 4,364 adult patients undergoing cardiac surgical procedures at this hospital from 1974 to 1978 have experienced bizarre and confusing postoperative courses, ultimately shown to be caused by acute adrenal insufficiency. Four were men and the age range was 53 to 73 years. None had evidence of preoperative Addison’s disease or endocrine hypofunction. In each instance, the operation was coronary artery bypass performed without untoward incident. Following uncomplicated postoperative courses ranging from 4 to 10 days, each exhibited insidious symptoms of flank or abdominal pain, delirium, fever, and eventual shock, occasionally preceded by hypertension. Diagnoses considered included leaking abdominal aneurysm, ischemic bowel, retroperitoneal hemorrhage, cholecystitis, pancreatitis, nephrolithiasis, stress ulcer, cecal volvulus, septicemia, brain tumor, cerebral edema, and schizophrenia. None had septicemia. All had abdominal surgical consultation and three underwent laparotomy. The correct diagnosis was first suspected on postoperative days 11 to 30 (average 19.0) and was proved on postoperative days 14 to 42 (average 21.0). Low serum cortisol levels strongly suggested adrenal insufficiency. Confirmation was based on lack of rise in urinary steroid determinations after 3 days of maximal adrenal stimulation with adrenocorticotropic hormone (ACTH). Four of the patients remain well receiving steroid replacement 5 to 60 months later. One died 16 months postoperatively of aortic dissection.