Risk factors, clinical courses, and urinary indices were examined in 12 consecutive patients with contrast media-induced acute renal failure. A high prevalence of preexisting renal disease, diabetes, vascular disease, and compromised cardiac output was observed. All patients had transient oliguria one to four days in duration, and all patients had return of renal function to their baseline values within five to ten days. Consistently low urinary sodium concentration and fractional excretion of sodium (FENa) were seen during the oliguric phase of the acute renal failure. Fractional excretion of sodium of less than 1% persisted for up to five days despite unequivocal renal function deterioration. The findings raise the possibility that acute renal failure secondary to contrast media may be mediated either by decreases in renal perfusion or by acute tubular obstruction.