This Diagnostic Seminar intends to announce that CMPDs can be classified from BMB histologically by a rather simple system, which can be applied by interested histopathologists successfully. The rationale of this classification is to stay within the groups of diseases which are outlined by clinical findings including the peripheral blood and bone marrow smears. The concept of traditional classification as given by the WHO and textbooks, however, has to be revised as follows (1) Primary diseases of CMPDs must be distinguished from advanced disorders. Primary diseases are CML, P. vera, Thrombocythemia, CMGM, and unclassifiable CMPD. (2) Idiopathic, primary myelosclerosis of the bone marrow is a reactive feature consecutive to neoplastic transformation of hematopoiesis, i.e. myeloproliferation. (3) Advanced disorders comprise (3.1.) excess of blasts and blast crisis, and (3.2.) early myelosclerosis, myelosclerosis and myelofibrosis, advanced myelofibrosis. Advanced disorders are designated by a composed term classifying them among the groups of primary disease and specifying the advanced stage by a suffix, so that the underlying disease remains coining the term, even in unclassifiable cases in which only CMPDs can be applied. (4) The CML group must be subtyped into CML of common type versus that with increase or predominance of megakaryocytes. By this system of classification, it seems possible to classify and type the spectrum of variations occurring among CMPDs to a satisfying result.