A series of 60 children subjected to urinary diversion is described. Particular emphasis is placed on the long-term results of 41 ileal conduit operations. The upper urinary tract became dilated postoperatively in 9 children in whom it had been normal preoperatively, dilatation increased in 1 child with only moderate dilatation preoperatively, and the degree of dilatation remained unchanged in a further 9 children who had preoperative dilatation. Operative and radiological findings suggested that stenosis of the ileal stoma or of the ureteroileal anastomoses was not the cause of the upper urinary tract dilatation which appeared in most instances only after a period of several years. Measurement of the pressure changes in the ileal conduit of 11 children selected at random showed that under normal circumstances intraluminal pressure remained low, but that obstruction of the stoma produced an immediate rise in pressure accompanied by strong ileal contractions. It is suggested that intermittent stomal occlusion caused by diversion appliances, clothing, or body posture may, by producing intermittent high pressure in the conduit, result over a period of years in gradual upper urinary tract dilatation. It is suggested that ureteroileostomy should not be used as a method of controlling urinary incontinence in children with normal upper urinary tracts, and that careful regard should be given to the unsatisfactory results of this operation in some children with deteriorating upper urinary tracts. It is possible that a sigmoid colon conduit may be more satisfactory than an ileal conduit.