A trial of the value of diuresis renography in equivocal upper urinary tract obstruction is presented. Fifty-two patients were examined by standard renography and renography under a diuretic provocation. The results indicate that the technique is of considerable value in making the vital distinction between dilatation on the excretion urogram due to atonicity and that caused by a genuine obstruction. A quantitative index of the response to diuresis reflecting the degree of impedance to flow is described--the Diuresis Excretion Index. Diuresis renography is recommended in the evaluation of the patient with equivocal urinary tract obstruction.