Ureteroceles are abnormalities which result from congenital stenosis of a uretero-vesical junction. That portion of the ureter which obliquely traverses the bladder wall becomes ballooned out so that it projects into the lumen of the bladder. This bulging mass in the bladder may be one to several centimeters in diameter. The lesions are more common in females. The outstanding symptoms are referable to pyuria, which appears in most cases. There may be abdominal or flank pain from the ureteric obstruction. In some patients the ureterocele may lodge in the bladder outlet and block the urinary flow from the entire urinary tract, in this way giving rise to damage in both kidneys. In rare instances a ureterocele may prolapse through the urethra of the female. Ureteroceles are apt to give rise to symptoms in infancy or childhood, but in some cases they are not manifest until later years. Intravenous pyelography and cystography will often give conclusive evidence of the presence of the abnormality, but in some patients cystoscopy will be necessary to establish the diagnosis. Ureteroceles are not difficult to treat. They should never be excised, because such surgical attack is apt to destroy the uretero-vesical valve and permit reflux of urine from the bladder to the upper urinary tract, a complication which results in increasing hydronephrosis and pyelonephritis. A ureterocele mass will collapse by merely incising its dome in a longitudinal manner for a distance of 2 to 4 mm. This effectively relieves the ureteral obstruction and is the most important step in the therapy of any existing urinary infection. Chemotherapy is a valuable adjunct in the treatment of pyuria, but will have permanent benefit only if the mechanical obstruction has been completely eradicated. In babies and small children it is preferable to avoid incision of a ureterocele by cystoscopic instrumentation; it is safer to open the bladder and incise the structure under direct vision. The results of this latter type of therapy are extremely gratifying and in most instances such conservative measures will permit saving of the kidney on the affected side.