Diseases of the bone marrow have a direct influence on osseous remodelling and can cause severe osteoporosis. Multiple myeloma, by way of the osteoclast activating factors produced by the pathologic plasma cells, regularly causes osteoporosis or, more frequently, osteolytic skeletal lesions (“skeletal-related events”, SRE). Polycythaemia vera (PV) (Fig. 44.1) and chronic myeloid leukaemia (CML) (Fig. 44.2) induce widespread osteoporosis by their expansive growth, but different histologic manifestations (Fig. 44.3). Changes in the bones also occur in osteomyelosclerosis (OMS) (Fig. 44.4) and during leukaemic transformation when increased levels of TNF-alpha and lactate dehydrogenase (LDH) are found. Not only the degree of cellularity but also the different proliferating haematopoietic cell lines influence osseous remodelling. While in PV, the trabeculae are attenuated but with normal microarchitecture and connections, a coarsening of the trabecular network with fewer connections and correspondingly large marrow spaces is characteristic of ML. Similar changes are produced by congenital haemolytic conditions, which cause extreme erythroid hyperplasia and osteoporosis.