After a short introduction on the advantages and limits of G.F.R. determination with Ccr, the radioisotopic methods proposed by Schlegel and Gates for this parameter are described, and as well as ERPF and FF, without blood serial samples being necessary. In a severe renal failure case due to obstruction, the Authors have found a great mismatch in the results between the traditional methods and the radioisotopic ones. After the clinical case description, a possible explanation of this discrepancy is proposed: Schlegel and Gates' methods, which well correlate Ccr in chronic renal failure cases, in severe renal failure on obstructive basis, of recent onset, could not indicate the effective glomerular filtrate, but the nephronic mass, functionally blocked by the endocapsular hypertension secondary to the obstruction, but anatomically unaffected and so recoverable by a timely irradication of the obstruction.