A previously healthy 39-year-old woman presented with severe iron-deficiency anaemia, but she had lost no blood and her serum iron level was high. Her bone marrow was hypercellular with a predominance of erythroid elements and had no stainable iron deposits, but it also showed dyserythropoiesis and an excess of apparently normal plasma cells. IgM was demonstrated on her bone-marrow erythrocytes and their precursors. On azathioprine and prednisone therapy she had a complete clinical and haematological remission. The impaired iron transport and the associated dyserythropoiesis were probably due to an IgM-mediated autoimmune process. Diabetes mellitus, which first appeared during her anaemic illness, could also have been due to an autoimmune process. This is the first report of an iron-deficiency anaemia caused by a naturally acquired impairment of iron transport.