We describe the first reported case of resistance to human recombinant erythropoietin (rhEPO) treatment caused by vitamin B12 deficiency in a chronic hemodialysis patient. Despite a normal B12 level before rhEPO treatment, resistant anemia together with a low B12 level and a megaloblastic bone marrow developed after only 8 months of rhEPO. There was a rapid reticulocyte response to B12 supplements, and transfusion requirements dropped from 2 units monthly to nothing. Atrophic gastritis was diagnosed through endoscopy and biopsy. Because of the fall in B12 level after 8 months of rhEPO treatment, we analyzed the results of routinely measured B12 levels in 30 hemodialysis patients treated with rhEPO, and found the mean B12 levels to be unchanged before and after rhEPO treatment. Although we found screening for B12 deficiency of little benefit, any patient with rhEPO resistance should have B12 levels tested, given the potentially serious extrahematological effects of B12 deficiency.