The patient was a woman in her 70's was referred to our hospital because of an abnormal shadow on chest roentgenogram at an annual medical checkup. Since preoperative examinations suggested lung cancer in the right middle lobe, thoracoscopic right middle lobectomy was planned. However, pleural dissemination was detected at surgery and we changed the treatment plan to the intrapleural hyperthermic chemotherapy. During the postoperative course, facial edema, hypokalemia, and hyperglycemia developed, and the diagnosis of Cushing's syndrome was suggested based on an increase in serum level of adrenocorticotropic hormone (ACTH) and cortisol, and was confirmed by a dexamethasone suppression test. Intrapleural hyperthermic chemotherapy was likely to collapse the ACTH-producing tumor leading Cushing's syndrome.