Of 12 patients with idiopathic edema, ten patients had received total doses of furosemide of up to about 950 g. A good negative correlation was observed between the creatinine clearance and the duration of daily oral furosemide intake of more than 40 mg, and a highly significant correlation was found between the logarithm of the creatinine clearance and the duration of daily furosemide ingestion. Cessation of furosemide and institution of a sodium-restricted diet was followed by improvement in the creatinine clearance. Three patients had acute renal failure with myoglobinuria. All patients examined showed tubular and/or interstitial changes. This study shows that in idiopathic edema, long-term furosemide treatment gradually impairs renal function, with reversal to a considerable degree after cessation of the drug, and that it causes organic changes in the kidney.