Among 750 patients who underwent surgery because of benign prostatic hyperplasia, 26 (3.5%) were found, before surgery, to have a huge residual urine of over 1,000 ml without classic prostatic symptoms. Dilatation of the upper urinary tract was found in 22 (84.6%) of these 26 patients. After surgery, almost two thirds of the patients remained with the anatomical damage (dilatation) of the upper tract. Before surgery all the patients had elevated serum creatinine. After release of the obstruction 50% of the patients were left with irreversible renal damage (as determined by serum creatinine). Although anatomical changes of the urinary bladder remained in all the patients, a satisfactory clinical recovery with a fairly good detrusor contraction and almost no residual urine was found after surgical removal of the obstruction. Awareness of the entity described here followed by early treatment can prevent progressive renal failure and detrusor decompensation.