Serum vitamin B-12 concentrations are measured to assess the presence of its deficiency in patients presenting with haematolological, neurological, and neuropsychiatric abnormalities. Replacement therapy is instituted promptly, particularly to prevent irreversible neurological and cognitive dysfunction. I present two cases with paradoxical vitamin B-12 results, which highlight the fallacies of the serum vitamin B-12 assay and emphasise the importance of taking into account the overall clinical picture before prejudging the significance of the vitamin B-12 assay result. ### Case 1 A 59 year old white woman was seen urgently for assessment of a macrocytic anaemia. She had normal serum B-12 concentrations, confirmed on three occasions. She complained of progressively increasing lethargy, palpitations, and buzzing in the ears over about three months. She had a good, well balanced diet and was not a vegetarian. Apart from thyroxine, she was taking no regular medication. She said her father had had pernicious anaemia. On clinical examination the only clinically significant findings were a mild glossitis and pallor. A full blood count showed a substantial macrocytic anaemia and a mild reduction of the white cell count (figure). The blood film showed mild oval macrocytosis, occasional nucleated red cells, and some hypersegmented neutrophils. An urgent bone marrow examination showed megaloblastic haemopoiesis.