13 patients with oliguric acute renal failure were given ethacrynic acid to induce diuresis. Histologic evidence of acute tubular necrosis was present in all patients who died. Seven patients responded with sustained diuresis, six patients did not. The most important factor determining the efficiency of treatment was the period of time between the onset of the oliguria and the time the ethacrynic acid was given. If this delay exceeded 24 h, only one patient responded; but all patients treated within 22 h responded. Permanent hearing loss in one patient and transient loss in another were the major side effects. Ethacrynic acid profoundly influences the natural course of acute tubular necrosis if given before 24 h of oliguria.