A patient with disabling postural tachycardia without postural hypotension had symptoms that included palpitations, weakness, abdominal and leg pain, light-headedness, headache and diaphoresis that occurred only in the upright position. She was shown to have an enhanced sympathetic neural response to standing (exaggerated plasma nor epinephrine response), and her cardiovascular responsiveness to released catecholamines was clearly intact. However, she was unable to maintain normal sodium balance and had a measurably reduced plasma volume while consuming normal amounts (120 mmol daily) of dietary sodium. Sodium loading (240 mmol ingested daily plus administration of fluorohydrocortisone, 0.1 mg daily) largely corrected the hemodynamic abnormalities, prevented postural symptoms and caused the compensatory sympathetic response to revert to normal.