In order to determine the antecedents of hypernatremic dehydration the authors reviewed the records of 56 patients with this condition at two public hospitals, one of which includes a large chronic care facility. Twenty-nine patients developed hypernatremic dehydration while at nursing homes. All cases came from proprietary nursing homes, although proprietaries account for only 88 per cent of nursing home beds in the community studied (P less than 0.05). There was a cluster of patients from two nursing homes. Sixteen patients admitted from home all showed evidence of inadequate care prior to admission. Eleven patients became hypernatremic while in acute care hospitals. No patient in the public chronic care facility developed hypernatremic dehydration during the period studied. The average serum sodium concentration of patients transferred from nursing homes was significantly higher than that of patients who developed hypernatremic dehydration at home or in acute care hospitals. It is concluded that hypernatremic dehydration in an institutionalized patient may be an indicator of inadequate care, which should prompt further investigation of the living conditions of the patient.