B owel is commonly used in urological surgery to replace the bladder, either as a conduit to drain urine to the abdominal wall as a uri na ry stoma or refashioned to fo rm a substitute bladder. H owe ver it is redeploye d, it still behaves as bowe l, e ven many ye a rs after transposition into the uri na ry tract, and complicat ions a rise because of its continuing secretory and absorp ti ve function. 1, 2 This behaviour has been the subject of a recent publication from Mainz in Germ a ny, where rat es of bladder substitution surgery are substantial. 3 The ileum and colon are the bowel segments most commonly used for bladder replacement. Both secrete sodium and bicarbonate and reabsorb ammonia and ammonium, hy drogen, and chloride ions when exposed to uri ne. 1 The ileum also absorbs potassium; the colon less so. 1 The consequence with either bowel segment is hyperchloraemic metabolic acidosis, 1, 2 mainly because of ammonium absorp tion. 4 In addition, there is tendency to total-body potassium depletion when colon is used. 3 The colon has more tight epithelial junctions than does the ileum, so there is a tendency for ileal segments to “leak” water down an osmotic gr adient. As a result, as much as a third of the total daily volume of urine may be the consequence of intercellular leakage of wat er across the ileal epithelium to maintain osmotic equilib rium rather than output from the kidneys. The more carefully metabolic acidosis is looked for, the more commonly it is found. In only about 17% of p atients is the venous blood hy perchloraemic, but in 50% or more there is metabolic acidosis on art e ri al blood-gas analysis. 2 In most of these patients this acidosis seems to be of little consequence, but bu ffe ri ng of this chronic acidosis in bone may lead to loss of gr owth potential5 and an increased incidence of o rthopaedic problems among gr owing children. 6 For this reason, gr owing children should always receive an alkalinising agent such as sodium bicarbonate to corr ect their metabolic acidosis after a uri na ry dive rs ion, whereas this preve nti ve measure could be considered desirable but not essential in adults. 4 When transposed to the uri na ry tract, a bowe l segment brings with it its reservoir of bacteri a. W hereas the urothelium of the normal bladder acts to maintain the sterility of the uri ne, intestinal epithelium does not; indeed it is accustomed to living in symbiosis with its bacte rial flora. And whereas a normal bladder is accustomed to distension, an intestinal substitute allow s transloc ation of bacteria from the mucosa to the systemic circulation when distended. As a result, both local and systemic infections occur more commonly after uri na ry dive rs ion, pa rticularly in patients with bladder substitutes. B acte ri u ria is found in up to 80% and acute infection of the uri na ry tract occurs in as ma ny as 20% of patients each ye a r. 1 This complication is probably the commonest cause of pat ients’ diss at is faction after an otherwise successful dive rs ion. B acte ri u ri a, coupled with the inevitable mucus excretion in the urine and a tendency to poor bladder emptying, which commonly requires clean interm ittent self cat hete ri s ation (CISC), m ay account for the increased incidence of stones when the bowel is re fashioned as a bladder substitute. The incidence is as l ow as 2% when the patient can void spontaneously bu t