The classification of blood system diseases is based on changes within individual groups of blood cells. They are most commonly defined as: diseases of erythrocytes, diseases of platelets and diseases of leukocytes. The diseases of erythrocytes are anaemias and polycythemia. Anaemia disorders resulting in oral symptoms and signs include iron deficiency anaemia, megaloblastic anaemia, haemolytic anaemia, hemoglobinopathies and aplastic anaemia. Manifestations include pale lips and pale mucous membranes, red, smooth and atrophic tongue, jaundice, deformations of maxilla, caries, gingivitis, dysphagia, paresthesias of mental nerve and others. Polycythemia is characterised by purple-red changes of the mucous membranes with bleeds. The diseases of platelets are thrombocytopenia and bleeding disorders. In patients with thrombocytopenia haemorrhage bullae, oedematous, hyperemic gingiva that bleeds spontaneously, as well as bad breath, may be found. Bleeding disorders resulting in oral signs and symptoms include haemophilia A, haemophilia B and Von Willebrand disease. The main clinical sign of these diseases is bleeding. The diseases of leukocytes are neutropenia and leukaemia. Oral manifestations of neutropenia are infections, periodontal diseases and ulcerations on the mucous membrane of the oral cavity. Oral manifestations of leukaemia occur due to a disease (gingival enlargement, destruction of the alveolar bone, teeth movement, pale mucous membranes, cyanosis, spontaneous bleeds, petechiae, ecchymoses, hematomas) or due to the treatment of a disease (mucositis, xerostomia, bacterial infections, fungal infections, osteoradionecrosis).