We report the frequency of the finding of storage and hemophagocytic histiocytes in the bone marrow of patients with systemic lupus erythematosus with one or more hemocytopenias. The study was performed on bone marrows of ten patients with systemic lupus erythematosus during an episode of hemocytopenia. Four patients were not receiving any treatment and six had been taking oral prednisone (3.5 +/- 1.5 mg/day) for the previous 6 months. Hemocytopenias found were lymphocytopenia in five, thrombocytopenia in three, and neutropenia and anemia in two each. The bone marrow had variable cell content; megakaryocytes, the myeloid:erythroid ratio, as well as lymphocyte, plasma cell, and reticular cell contents were usually increased. Seven bone marrow samples showed storage histiocytes (sea-blue histiocytes and other histiocytes that contained unidentified intracytoplasmic material). These same samples revealed histiocytes phagocytosing erythoblasts, erythrocytes, polymorphonuclear neutrophils, lymphocytes or platelets. The presence of these histiocytes in all seven patients was related to a decrease in serum complement and with lupus disease activity as measured with the SLEDAI index (mean +/- SD = 2.1 +/- 1.5). The SLEDAI score of the remaining three patients, who had no histiocytes in their bone marrow, was 0, 0, and 1, respectively. The present study reveals that the bone marrow in patients with systemic lupus erythematosus and peripheral hemocytopenia contains storage and hemophagocytic histiocytes and the significance of these cells is discussed.