The syndrome of inappropriate antidiuretic hormone (SIADH) secretion is a common consequence of neurologic and pulmonary infections as well as drug intake and many other clinical situations. This report describes SIADH that developed in an elderly woman with single dermatomal herpes varicella zoster ophthalmicus without evidence of varicella zoster encephalitis or dissemination. A 76-year-old woman was admitted to our department for evaluation of left facial pain, confusion and disorientation. Further investigation revealed hyponatremia 112 mEq/L, low serum osmolality, high urine osmolality, normal renal function, normal adrenal and thyroid hormones, and high plasma vasopressin 40 pg/mL. These results indicate that the hyponatremia in this case was due to SIADH and that SIADH was caused by an increased release of vasopressin probably because of the antiviral drug (acyclovir) or infection of varicella zoster virus(VZV)in a single dermatome.