Severe hyponatremia developed within 2 weeks of head injuries in three elderly patients. Before the head injuries occurred, normal serum levels of sodium had been found in two of the three patients. Hyponatremia (105 to 117 meq/L) was associated with persistently increased urinary excretion of sodium. The patients appeared dehydrated and had lost weight. The mean plasma level of antidiuretic hormone was 5.0 +/- 1.6 (SD) pg/mL, which was relatively high despite decreased osmolality. Plasma renin activity was suppressed to 0.25 +/- 0.13 ng/mL X h, and plasma aldosterone levels measured low-normal or normal. Plasma renin activity and plasma aldosterone levels remained unchanged after the patients were given furosemide and placed in an upright position. The hyponatremia promptly resolved after the administration of fludrocortisone acetate, 0.1 to 0.4 mg/d. These observations indicate that severe hyponatremia occurs in elderly persons rapidly after head injuries, that it responds well to mineralocorticoid hormone therapy, and that both central nervous system and renal components may be involved in the mechanisms of action of the disorder.