RESUMEN
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the term applied to arginine vasopressin (AVP) excess associated with hyponatremia without edema in the absence of physiologic or pharmacologic stimuli to AVP secretion. SIADH is associated with various conditions such as malignant tumors, infection, central nervous system disorders, and different pharmacological agents. The patient was 73-year-old female. She was admitted to the hospital because of persistent cough, dizziness, general weakness and confusion. On admission, her serum osmolality was 253 mOsm/kg, urine osmolality was 416 mOm/kg, and urine Na concentration was 159 mEq/L. Her Chest X-ray and CT scan of lung showed about 4x3.5 cm sized mass at posterior basal segment of left lower lobe of the lung, and CT-guided percutaneous needle aspiration revealed small round cell with clusters of malignant squamous cells. She was treated by salt restriction, hypertonic saline infusion and demeclocycline. We planned chemotherapy for advanced combined lung cancer, but she was discharged because of poor general condition and associated pneumonia without cancer chemotherapy. We report a rare case of SIADH in small cell cancer of lung combined with squamous cell cancer of lung.