Obstruction of urinary tract is a frequent cause of transient renal dysfunction, which, in the majority of cases, is reversible. Urinary obstruction can occasionally cause chronic renal failure, in spite of successful relief of the obstruction. Sonographic and pathologic studies show renal atrophy, interstitial fibrosis, and interstitial inflammation, which explain the loss of renal function. Herein, we report two cases of bilateral obstructive nephropathy due to prostatic hyperplasia, in which renal ultrasound demonstrated hydronephrosis and normal cortical thickness. In both cases, the renal failure persisted even after urinary obstruction was relieved by continuous catheter drainage. Renal biopsy showed that in each case the cause of the irreversible renal failure was chronic tubulointerstitial nephritis. It is concluded that renal biopsy can be of diagnostic, as well as of prognostic value in obstructive nephropathy, especially when relief of the obstruction is not followed by restoration of the renal function.