It is seldom realized that at least 90 per cent of major urologic problems are related to urinary tract obstruction, dysfunction, and infection. The importance of this is reflected in the vigorous campaign, waged by urologists for years, to salvage functionally impaired renal tissue. Today, “renal salvage” is becoming increasingly important as the incidence of trauma mounts. To save even a third of a kidney may be to save the tissue which preserves life in a future accident in which the opposite kidney is destroyed either temporarily or permanently. The most common site for supravesical congenital obstruction of the urinary tract is the ureteropelvic junction. Interest in determining the cause of such obstruction was stimulated in the course of an effort to establish a consistently successful corrective surgical procedure. This study will present a heretofore unrecognized cause of obstruction, with a description of an operation that has been unusually successful in re-establishing a morphologically and physiologically normal ureteropelvic area. Obstructions of the ureteropelvic junction are not uncommon, but they vary greatly in degree. In a review of 250 consecutive retrograde pyelograms, some evidence of pyelectasis, unilateral or bilateral, was found in 76.2 per cent, though in not a single instance was this clinically significant. Our interest lies in those cases in which the obstruction is sufficient to cause not only pyelectasis, but also calyceal blunting and retention of urine beyond the usually acceptable ten-minute interval. The two problems studied were: (a) the cause of obstruction and (b) its surgical correction. Henline and Menning pointed out, some years ago, that the prevalent concept of roentgenologists and urologists alike was that ureteropelvic obstruction was due to an extrinsic cause, i.e., aberrant vessels, fibrous bands, etc. Actually, these abnormalities are only rarely responsible. In Henline's own experience, less than 50 per cent of the patients treated by division of these seemingly obstructive structures had satisfactory renal drainage post-operatively, and the incidence of secondary nephrectomy for progressive hydronephrosis was at least 60 per cent. Such reports in the literature, in addition to our own observations, led to the initiation of this study almost twenty years ago. In view of repeated instances in which the resection of an apparently obstructing band or vessel was valueless in relieving ureteropelvic obstruction, it was quite apparent that the cause of obstruction must be sought elsewhere. Furthermore, if one attempted to reconstruct the picture with the renal pelvis in its initial non-dilated state, the location of the obstructing band or vessel would seemingly be sufficiently far removed to be of no obstructive significance.