1. On constant high (200 mmol)- and low (10 mmol)-sodium diets, thirty-one patients with idiopathic oedema have been compared with six non-oedematous control subjects while upright from 08.00 to 20.00 hours for 3 days and while recumbent all day for 2 days.
2. The cumulative weight rise in the upright posture and fall in recumbency were ‘normal’ in five patients (‘non-orthostatic oedema’) and excessive in twenty-six (‘orthostatic oedema’).
3. Leg volume showed a cumulative rise during the upright period, which was excessive in the orthostatic oedema patients only.
4. When the orthostatic oedema patients were in the upright posture, fifteen retained sodium excessively (‘orthostatic sodium retainers’), while eleven retained ‘normal’ amounts of sodium (‘orthostatic water retainers’).
5. The ‘orthostatic sodium retainers’ experienced excessive acute orthostatic falls in inulin and para-aminohippurate (PAH) clearances. Half of them showed excessive aldosterone excretion and secretion rates and excessive potassium excretion in the upright posture on the 200 mmol of sodium diet. Plamsa volume fell excessively after 1 h in the upright posture in these patients. None of these changes was significant or consistent in the other patients with oedema.
6. Plasma renin activity and plasma protein and albumin concentrations were ‘normal’ in all patients in all circumstances.
7. Idiopathic oedema is a group of disorders each associated with: (I) orthostatic fall in filtered sodium often with orthostatic hyperaldosteronism and consequent orthostatic sodium retention; (II) orthostatic water retention possibly related to orthostatic hypervasopressinism and occasionally associated with occult heart failure; or (III) non-orthostatic factors.