We report here a case of advanced gastric cancer complicated by Syndrome of Inappropriate Secretion of Antidiuretic Hormone（SIADH）following staging laparoscopy. A ５５ year-old male developed nausea, and was found with a poorly differentiated adenocarcinoma of stomach（UE, type4, cT 3, cN2, cM0, cStage ⅢB）. He was done with staging laparoscopy. The serum sodium concentration decreased from１３８mEq/l to１１４mEq/l after operation. SIADH was diagnosed on the basis of hyponatremia with corresponding serum hypoosmolality and an inappropriate high urinary osmolality due to continued sodium excretion. Fluid restriction and sodium supplement resulted in an appropriate rise in the serum sodium level to１２８mEq/l in ４ days
