Histological investigation in 31 chronic myeloid leukemia (CML), 29 CMPDs not otherwise specified (CMPD-NOS), 28 essential thrombocytosis (ET), 15 polycythemia vera (PV) and three chronic eosinophilic leukemia/hypereosinophilic syndrome (CEL/HES) exhibited in 32% various amounts of lymphocytic infiltrate of sparsely to moderately diffuse or nodular types in the bone marrow, but the reactive or coinciding lymphomatous nature could not be revealed by histology alone in the majority of cases.