Idiopathic edema is a syndrome of real or perceived excessive weight gain. This article reviews what is known about the possible causes, evaluation, and treatment. Although the cause is unknown but often thought to be due to secondary hyperaldosteronism, primary abnormalities of the hypothalamus, thyroid, dopaminergic release or renal dopaminergic metabolism, vascular basement membrane, or capillary sphincter control could perhaps contribute in some patients. The diagnosis requires careful attention to possible abnormalities of the liver, heart, kidneys, gastrointestinal tract, thyroid, and pancreas. The history must include an evaluation for risks of bulimia and purging; diuretic and laxative screening should be performed. Specific records of daily weights, urinary outputs, and menstral cycle dates are useful. Treatment may include dietary counseling to provide weight control and a constant carbohydrate intake, treatments for depression, compression stockings, spironolactone, amiloride, angiotensin II inhibitors, or sympathomimetic agents, depending on the severity and timing of the patient's symptoms. Unfortunately, idiopathic edema may be a multifactorial disorder that has not been completely delineated. Further research into possible causative mechanisms is required before a more useful algorithm for evaluation and treatment is available.