Classification of non-Hodgkin's lymphomas in bone marrow biopsies may be hampered by several factors, including histotechnical artifacts. The authors compared the initial morphologic characteristics of B-cell lymphomas and leukemias in uniformly plastic-embedded biopsies of lymph nodes and bone marrow, using the Kiel classification and International Working Formulation (WF). Fifteen of 48 cases with multiple positive biopsies showed major differences, with 9 cases resulting in a discrepant malignancy grade. A discordant low grade was found in the bone marrow of seven cases. Two patients with rapidly progressive bone marrow insufficiency had a reversed pattern. One patient with a composite lymphoma also had disseminated high-grade lymphoma. In 4 of 22 patients with a follicle center cell lymphoma, bone marrow histologic characteristics suggested immunocytoma. The available data suggest that most differences between bone marrow and lymph node biopsies are caused by tumor heterogeneity. Morphologic evaluation of this heterogeneity may be of help in the treatment of individual patients.