Carbamazepine (CBZ) is a drug which can induce the syndrome of inappropriate antidiuretic hormone (SIADH). Until 1980's, there were reports regarding CBZ-induced SIADH, but it is rarely reported these days. We here report two cases of CBZ-induced SIADH. Hyponatremia in these cases was rapidly improved by withdrawal of administration of CBZ. According to the previous reports, the rate of hyponatremia in patients receiving CBZ is not small. It ranges from 48% to 31%. As CBZ is frequently used for patients with epilepsy and neuralgia, not only their blood CBZ concentration but also their serum Na level should be monitored.