We report the first case of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) associated with cytomegalovirus (CMV) infection. A 32-year-old woman was admitted to our hospital because of pandysautonomic signs and symptoms. Thrombocytopenia and hyponatremia were present. Serum anti-CMV IgM and IgG antibodies were positive. Despite hyponatremia, urinary osmolality exceeded plasma osmolality and plasma vasopressin levels related to plasma osmolality were high. Restriction of water intake and administration of dimethylchlorotetracycline improved hyponatremia, suggesting this patient had SIADH. In this patient, SIADH may have been caused by acute pandysautonomia that developed following CMV infection.