A clinical and investigative study is reported of 19 patients with 'idiopathic oedema of women'. The resons for defining this as a specific syndrome unrelated to the menstrual cycle are given, and the clinical features reviewed. During a forced water diuresis the flow and composition of the urine and the plasma volume were studied on tilting from the supine to the upright position seven premenopausal and four postmenopausal patients with this disorder. No differences were found in the results obtained in the follicular and luteal phases of the menstrual cycle or in the pre- and post-menopausal patients. The reductions in urinary volume and electrolyte excretion on upright tilting were greater than those observed under similar circumstances during the luteal phase of the menstrual cycle in normal female controls, and attributed to increased proximal renal tubular reabsorption. The rate of loss of isotopically labelled albumin from the intravascular compartment was greater in patients with idiopathic oedema than in control subjects. A reduction in blood volume on tilting occurred in control subjects and patients with idiopathic oedema, but was greater in the latter; and the larger the fall, the greater were the reductions in urinary flow and electrolyte excretion. The effect of administering 9-alpha-fluorohydrocortisone was studied in nine patients with idiopathic oedema. One patient failed to 'escape' from the sodium-retaining action of this mineralocorticoid and developed pulmonary oedema; the others 'escaped' normally. The pathophysiological disturbance in this condition is related to increased loss of fluid from the vascular compartment but the precise aetiological mechanism remains unknown.