Paroxetine is a selective serotonin reuptake inhibitor (SSRI) widely used for depression in brain damaged patients as it is known for its few affects on cognition or psychomotor function. Major side effects observed for paroxetine are vertigo, headache and tremor. It is reported that hyponatremia is also rarely observed. In 69 year-old male with left hemiplegia due to cerebral infarction had started taking 20 mg/day of paroxetine for depression. Next day the patient showed severe generalized weakness and decreased consciousness. Laboratory tests showed that serum sodium level was decreased to 102 mEq/L. Paroxetine administration was stopped and 3% sodium and oral furosemide were administered under the diagnosis of syndrome of inappropriate secretion of antidiuretic hormone (SIADH). We report a case of patient with severe hyponatremia due to SIADH within 2 days after use of paroxetine. This case emphasizes the need to monitor serum sodium level routinely at the initial treatment to prevent hyponatremia.