Our aim was to determine the frequency of orthostatic edema (OE) in patients with idiopathic intracranial hypertension (IIH). We evaluated 30 women with IIH for evidence of OE by comparing sodium and water excretion in the recumbent and standing postures and morning and evening body weights. Data were compared with findings in 30 women with OE, 22 weight-matched obese normal subjects, and 20 lean normal subjects. The effect of treatment with diuretics or diuretics plus sympathomimetic agents was compared. Seventy-seven percent of IIH patients had evidence of peripheral edema and 80% had significant orthostatic retention of sodium or water. Excretion of a standard saline load and of a tap water load was significantly impaired in the upright posture in the IIH and OE patients compared with the lean and obese normal subjects. Diuretic therapy induced weight loss (up to 9 kg) and decreased mean weight gain from morning to evening in 5 of 12 patients treated. In seven patients also treated with diuretics plus sympathomimetic drugs, the diuretic-induced morning weight loss and morning to evening weight gain were both significantly improved with the addition of sympathomimetic agents. Therapy reduced the frequency or severity of headaches in seven patients and reduced papilledema in four patients who received no other concurrent treatment for IIH. The orthostatic retention of sodium and water and the consequent edema is very similar in IIH and OE patients, suggesting a common pathogenesis for both disorders. Diuretic therapy, dietary salt and water restriction, and planned periods of recumbency merit study as a treatment for these patients.