Diuresis renography is widely used to distinguish obstructed from nonobstructed kidneys; however, the delivery of furosemide to its site of action in the loop of Henle is impaired in patients with azotemia. Consequently, the standard adult dose of 40 mg furosemide could be insufficient to generate an adequate diuretic response. This problem is illustrated by a patient with azotemia with bilateral nephrostomies who underwent Tc-99m MAG3 (mercaptoacetyltriglycine) diuresis renography with 40 mg furosemide to determine if his bilateral ureteral obstruction had resolved. The study showed findings typical for obstruction despite the fact that the patient could not have been obstructed because the nephrostomy tubes had not been clamped. When the study was repeated 6 days later with 80 mg furosemide and clamped nephrostomy tubes, there was good drainage bilaterally excluding obstruction. The nephrostomy tubes were removed and the patient's creatinine has subsequently remained stable for 3 years. In summary, this report illustrates the rationale for increasing the dose of furosemide in patients with azotemia referred for diuresis renography and shows how increasing the dose of furosemide could improve the diuretic response and minimize false-positive or indeterminate results.