Ten cases of epithelioid granulomas in the bone marrow of patients with various non-Hodgkin's lymphoproliferative malignancies have been encountered. These included six with non-Hodgkin's lymphoma (three histiocytic and three poorly differentiated lymphocytic types), three with multiple myeloma, and one with acute lymphoblastic leukemia. The bone marrow was not involved by the primary disease in two of the six patients with lymphoma, whereas three with lymphoma showed both granulomatous and lymphomatous lesions in the same marrow specimens, and in one, these lesions were seen in the marrow at different times. The three myeloma patients showed evidence of both myeloma and granulomas in their marrow. In the case of acute lymphoblastic leukemia, the bone marrow showed only granulomas, the leukemic process being in complete remission. Although small numbers of similar cases have been reported before, the authors were unable to find a previous report of acute lymphoblastic leukemia associated with bone marrow granulomas. Although the pathogenesis and the clinical significance of the granulomatous lesion of the bone marrow in non-Hodgkin's lymphoproliferative malignancies are unknown, this lesion should be differentiated from infectious or lipid granulomas as well as from involvement by the primary disease.