The results are presented of the first long-term follow-up study of patients with suspected obstruction of the upper urinary tract investigated by a modified form of diuresis renography (F - 15 renography). The incidence of equivocal results from diuresis renography is significantly reduced by the use of the F - 15 renogram. F - 15 renography provides a reliable assessment of the upper urinary tract in patients with suspected obstruction and is recommended in the investigation of equivocal upper urinary tract dilatation.