Aetiology, pathogenesis, clinical symptoms, diagnosis, prophylaxis and therapy of neonatal isoerythrolysis in foals are presented. Neonatal isoerythrolysis is caused by isoimmunisation of a brood mare to the Aa and Qa erythrocyte antigens of the foal. The disease can develop, when the mare does not possess Aa resp. Qa blood group antigens, is sensitized to the Aa or Qa erythrocyte antigens--i.e. through pregnancy, parturition, blood resp. plasma transfusions, etc.--and the foal suckles colostral antibodies to its own blood cells. Aa and Qa antibodies can cause haemagglutination and haemolysis in the foal, with a consequent decline in erythrocytes, PCV and haemoglobin resulting in several clinical symptoms. In most instances the first signs of the disease are noticed by day 2 and 3, ranging from 8 to 96 hours of life. Diagnosis is based upon clinical examination and determination of erythrocyte count, PCV and haemoglobin concentration and can be further confirmed by immunological tests. Several tests can be used to prevent the occurrence of neonatal isoerythrolysis in the newborn foal. Prior to parturition, brood mares can be typed for blood groups and tested for antibodies to Aa and Qa in order to identify mares at risk for causing neonatal isoerythrolysis in the foal. After birth, compatibility of the mare's colostrum and the foal's erythrocytes can be checked by the "jaundice foal agglutination" test. Some instructions for prophylaxis of neonatal isoerythrolysis and for the treatment by red blood cell resp. whole blood transfusions are given.