A case of syndrome of inappropriate secretion of antidiuretic hormone (SIADH) associated with Ramsay Hunt syndrome is reported. A 59-year-old man was admitted to our department for treatment of left facial pain that had persisted for three days. A left renal tumor had been diagnosed and a radical nephrectomy had been performed two months earlier. On admission, vesicular lesions were found in the left external auditory canal and ear lobe. Additionally found were left facial nerve palsy and hearing loss. Acyclovir 5 mg/kg, three times per day, was started. Five days after admission, the patient became confused and disorientated. Further investigation revealed hyponatremia 118 mEq/l, low serum osmolality, high urine osmolality, normal renal function, normal adrenal and thyroid hormones, and high plasma vasopressin 30 pg/ml. Although cerebrospinal fluid (CSF) examination revealed a mild elevation in protein and cells, no malignant cells were present and bacterial examinations were negative. Antibodies to varicella-zoster virus (VZV) in both IgG and IgM were present in high titers not only in the serum but also in the CSF. Intravenous hypertonic saline and water restriction were started, and the patient's sensorium was improved in accordance with the increase in serum sodium concentration. 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 infection of VZV in the central nervous system.