Human bone marrow is a fascinating and complex organ. Its major function is to produce and sustain normal haemopoiesis, and in a normal adult this entails the production of more than 1011cells/day. The marrow contains most of the multipotent haemopoietic stem cells, and provides an environment for these cells to differentiate into each of the well recognised peripheral blood cells (erythroctes, leucocytes, and platelets). In addition, the marrow nurtures the development of B lymphocytes and provides T cell progenitors which migrate to the thymus and differentiate there into mature T cells. This brief review is selective and focuses on the neurological aspects of several bone marrow disorders—namely, multiple myeloma, paraproteinaemia, Waldenstrom’s macroglobulin, cryoglobulinaemia, and lymphoma. Neurological aspects of bone marrow transplantation and chemotherapy are also considered. Multiple myeloma is the most common of the plasma cell dyscrasias, which also include monoclonal gammopathies of unknown significance (MGUS or paraproteinaemias), plasmacytomas, and plasma cell leukaemia. These terms represent a range of diseases characterised by a monoclonal proliferation of plasma cells, and associated with a corresponding diversity in clinical behaviour. Antibody molecules, the product of plasma cells, are composed of two heavy and two light chains. There are five heavy chain isotypes (G, M, A, D, and E) and two light chain isotypes (κ and λ). Each chain consists of a constant region and a variable region. The second provides the recognition site for the antibody molecule; its structural uniqueness or idiotype derives from a particular clone of cells as each antibody is produced by a single clone. Serum electrophoresis produces a broad peak in the γ region, which is composed of a very large number of immunoglobulin molecules each specified by a unique plasma cell clone. In patients with multiple myeloma and other plasma cell dyscrasias a sharp spike or …