In most cases megaloblastic anaemias are the sequel of a deficiency of vitamin B12, more infrequently of a deficiency of folic acid of different etiology. Oriented to frequency and anamnesis the diagnostics follows the leading symptoms of pernicious anaemia (straw colour, glossitis, achlorhydria) and on the basis of special findings in the peripheral blood (hyperchromacia, megalocytosis, much decreased number of reticulocytes, increased iron and bilirubin level) it leads to the proved suspicion of a megaloblastic anaemia. This suspicion is ascertained by the investigation of the bone-marrow, with the help of aimed investigations the anaemia is further clarified differential-diagnostically. An unclear anaemia should not be treated ex juvantibus with vitamin B12 and/or folic acid. The therapy, always taking into consideration a possible basic disease, is carried out by parenteral application of vitamin B12, possibly in form of hydroxocobalamine or by folic acid. In persisting disturbance of the resorption of vitamin B12 on account of the threatening complication of a funicular spinal disease the long-term therapy must never be interrupted, unless in normal haematological findings.