BACKGROUND: Vitamin B12 is absorbed exclusively in the terminal ileum the resection of which may produce malabsorption of B12. The present study aimed to determine whether the length or specific segment of the intestine used in bladder reconstruction affects the overall incidence of B12 malabsorption. It was also aimed at the reasonable way of following these patients. METHODS: Seventy patients who underwent urinary reconstruction between 1988 and 1997 were studied: 28 had undergone orthotopic reconstruction, 14 had ileal conduit diversion, 8 had continent diversion and 20 had undergone enterocystoplasty. The median follow up was 65 months (10-137 months). Indications for operation included carcinoma of the bladder, neurogenic bladder dysfunction, idiopathic detrusor instability and interstitial cystitis. Schilling tests were performed, and serum B12 level and haematological indices were measured. RESULTS: Patients were classified depending on whether the ileocaecal junction was used. Group 1 included ileal conduit and entero-cystoplasty where 15-20 cm of ileum, 15-20 cm from ileocaecal junction was used. Group 2 consisted of orthotopic reconstruction and continent diversions where the ileocaecal segment was used. Group 3 comprised patients in whom long ileal segments (50-60 cm) had been used for reconstruction. Five patients in group 1 and one in group 2 had low B12 levels, but none had developed neuropathy or megaloblastic anaemia. One patient in group 1, and six patients in group 2 had low Schilling tests indicating intestinal malabsorption of B12. No patient in group 3 had a low B12 or an abnormal Schilling test. CONCLUSION: This study showed that use of ileocaecal segments results in intestinal malabsorption of B12. The length of ileum alone does not seem to be the determinant factor in causing B12 deficiency or B12 malabsorption.