Vitamin B12 deficiency was found in 10 of 41 patients who underwent radiotherapy before cystectomy with Bricker urinary diversion for carcinoma of the bladder. Of 13 patients given full irradiation because of inoperable bladder cancer 5 had malabsorption of vitamin B12. Serum folic acid was normal in these patients, indicating predominantly ileal irradiation sequelae. Routine evaluation of serum vitamin B12 after radiotherapy is recommended so that appropriate medication can be given, if possible before neurological symptoms appear.