A retrospective review of 27 patients with nonvisualization on 131I-orthoiodohippurate (Hippuran®) renal scan during 1972–1977 was carried out. 5 patients had renal failure caused by urinary tract obstruction and of these, 4 were submitted to surgical relief. Varying levels of life-sustaining renal function were recovered in all 4 patients. 16 had chronic intrinsic renal disease, of whom 7 were admitted for reasons not directly related to renal failure. All of these required chronic dialysis within 3–6 months. The remaining 7 patients had acute renal failure (clinically, acute tubular necrosis) and none of them survived. It has been well established that the prognosis for recoverability of renal function is extremely poor in patients with nonvisualization on hippuran scan. It is important, therefore, to emphasize that nonvisualization on 131I-orthoiodohippurate renal scan in patients with urinary tract obstruction does not exclude the potential for recoverable renal function. Therefore, even in the absence of renal visualization, the need to definitively rule out urinary tract obstruction rem