We present the case of a 74-years old man with subtotal gastrectomy for adenocarcinoma, whose postoperative course is complicated by a prolonged fever. Despite extensive diagnostics, the origin of the postoperative fever remains unexplained for 47 days, when the patient is transferred to the ICU because of lethargy, hemodynamic instability, lung edema and abdominal symptoms. The diagnosis of Addison crisis is established and under substitution with hydrocortisone all the symptoms rapidly resolve. Postoperative fever is relatively frequent and infectious causes account for about half of the cases only. An infectious origin has imperatively to be excluded when the fever arises later than 48 hrs after surgery. In case of prolonged fever we suggest to extend the differential diagnosis to other than infectious causes by means of special lists (e.g fever of unknown origin).