Treatment strategies for hyponatremia such as hypertonic saline, normal saline with furosemide, urea, fluid restriction and demeclocycline are well established. However, these treatment modalities may themselves be associated with life-threatening complications. An important complication is rapid correction of hyponatremia with its consequent serious neurologic deficits and death. An unrecognized complication is the development of severe hypernatremia as a result of strict fluid restriction and concomitant excessive free water excretion from prolonged outpatient demeclocycline therapy. The authors report a case of a patient with hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion who developed severe hypernatremia as a result of rigid fluid restriction and demeclocycline therapy.