The aim of this investigation was to study the validity of the radionuclide methods in the estimation of kidney function, for prognosis and follow-up of acute renal failure (ARF). In thirty-one ARF patients, the evaluation of glomerular filtration rate (GFR) by 99mTc-DTPA clearance and effective renal plasma flow (ERPF) by 131 I-ortoiodohippurate (131I-OIH) clearance was performed within 7 days and after 6 months from ARF onset. All patients were divided in three groups according to 131I-OIH clearance values obtained within 7 days: group 1, under 150 mL/min; group 2, 150-250 mL/min; and group 3, over 250 mL/min. Seven days clearance values of both radiopharmaceuticals were found to be very low, however, GFR was found more severely impaired than ERPF. Clearance values obtained after 6 months demonstrated no recovery of renal function in the first group, partial recovery in the second and almost complete recovery in the third group. Patients with the lowest 131I-OIH clearance values at the ARF onset had no recovery of renal function, while in the other two groups recovery corresponded to initial 131I-OIH clearance values. In patients with ARF both, 99mTc-DTPA and 131I-OIH clearances were shown suitable for the follow up of renal function, however, only 131I-OIH clearance had a strong predictive prognostic value for renal function recovery in ARF.