Increased numbers of bone marrow mast cells were found in 45 (2.2%) of 2,000 bone marrow specimens obtained from patients who had hematologic disorders. Mast cells were most frequently seen in the marrows of patients who had preleukemic syndromes, lymphoproliferative disorders, and acute leukemia. The 16 patients who had preleukemic syndromes included those with refractory sideroblastic and megaloblastic anemia (with or without an excess of blasts), idiopathic pancytopenia or pure erythrocytic aplasia, paroxysmal nocturnal hemoglobinuria, idiopathic refractory neutropenia, agranulocytosis or thrombocytopenia, and persistent eosinophilia. Five of the seven patients who had acute leukemia had nonlymphoblastic leukemia; two had blastic crisis of chronic granulocytic leukemia. Of the 13 patients who had lymphoproliferative disorders, eight had chronic lymphocytic leukemia, three had macroglobulinemia, and two had non-Hodgkin's lymphoma. Three patients who had chronic renal failure associated with severe anemia and two who had chronic liver disease, splenomegaly, or hypersplenism were also encountered. In this study there appeared to be a consistent relationship between the presence of increased numbers of mast cells and the lymphocyte and plasma cell counts in the bone marrow. The significance of the presence of secondary mastocytosis in premalignant lesions, neoplasia, and, in particular, lympho- and myeloproliferative disorders, is still unclear.