With the description of facial abnormalities in haemolytic anaemia, Cooley in 1927 first described the relationship of hyperplastic erythropoiesis and increased bone remodelling with malformation of the skeleton. Especially in severe congenital haemolytic anaemias, such as sickle cell anaemia, thalassaemia major and hereditary spherocytosis, there are classic but quite similar osseous abnormalities due to marrow expansion (“haematic dysplasias”). The whole bone may be affected, but the most prominent changes occur in the axial skeleton homing the red marrow. Abnormalities are striking in the skull with widening of the diploic space and thinning of the outer table. Radial spicules of the bone lead to a “hair-on-end appearance” or “brush head”. Osseous expansion also occurs in the face and causes striking facial abnormalities. Also vertebral body deformities are common, with wide marrow cavities and thin cortices. There is, however, no close correlation between the severity of the anaemia and osseous abnormalities on skeletal X-ray. Local lesions are also caused by microvascular occlusions (aseptic necrosis, infarcts and osteomyelitis).