The bone marrow examination is useful in the diagnosis and management of patients with malignant lymphoma. Demonstration of marrow lymphoma is dependent on obtaining adequate and properly prepared tissue for examination. The incidence and pattern of marrow disease differ for the various Working Formulation classes of non-Hodgkin's lymphoma (NHL); low grade and high-grade NHL have the highest incidence of marrow involvement at diagnosis. Hodgkin's disease involves the marrow less commonly than NHL and is found in a histologic environment similar to that in lymph node biopsies. Several benign lesions, including reactive lymphoid aggregates, granulomas, immunoblastic lymphadenopathy, and mast cell disease, must be distinguished from NHL or Hodgkin's disease in some instances.