The terminal ileum is widely used for reconstruction of the lower urinary tract. Since the terminal ileum exclusively absorbs vitamin B12, removal of ileum from the gastrointestinal trace for use in lower urinary tract reconstruction may predispose patients to vitamin B12 deficiency. In a prospective study serial vitamin B12 levels were obtained postoperatively in 24 patients who underwent cystectomy for malignancy and ileocolic neobladder urinary diversion with a mean followup of 25 months (range 6 to 53). Any patient who had a low vitamin B12 serum level (less than 200 pg./ml.) underwent a Schilling test to confirm malabsorptive vitamin B12 deficiency. Six patients (25%) had low serum vitamin B12 levels, of whom 3 (13%) had an abnormal Schilling test. No patient had megaloblastic anemia. One patient with malabsorptive vitamin B12 deficiency had neurological symptoms 53 months postoperatively. We conclude that vitamin B12 deficiency can occur following ileocolic neobladder reconstruction. Patients with true vitamin B12 deficiency should be identified and placed on lifelong parenteral vitamin B12.