Extensive bone marrow infiltration by Hodgkin's lymphoma. On trephine biopsy, there is replacement of normal hematopoietic cells by Reed-Sternberg and mononuclear Hodgkin's cells, surrounded by small lymphocytes and a fibroblastic reaction. Bone marrow aspirate shows infiltration by mononuclear Hodgkin's cells. (Courtesy of P.T. Murphy and C.S. Chapman, Department of Haematology, Leicester Royal Infirmary, Leicester, UK.)