In temperate climates deficiency of vitamin B12 is usually due to defective absorption, most commonly from lack of intrinsic factor as in Addisonian pernicious anaemia and after gastric surgery; occasionally the absorptive defect is caused by gastric carcinoma, resection, short-circuiting or disease of the ileum, including an abnormal bacterial flora, or by the fish tapeworm (Diphyllobothrium latum). The effectiveness of oral therapy remains debatable,1 and, since the absorptive defect can rarely be corrected, injections of vitamin B12 are virtually essential.