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In 1,083 core biopsies of the bone marrow with myeloproliferative diseases 454 cases or 42% were found to have neoplastic megakaryopoiesis. Neoplasia of megakaryocytes was assumed from the conspicuous cytological atypicality revealed by light microscopy, extending and confirming earlier ultrastructural findings. Histopathology of the bone marrow in these patients was described as chronic megakaryocytic-granulocytic myelosis - CMGM - since neutrophilic granulopoiesis is also apparently neoplastic and both cell lineages showed a complete differentiation to mature forms. CMGM should be separated from the chronic granulocytic leukemia - CGL - which consists of only a single line proliferation. The incidence of CGL in our total of 1,083 patients was 25%. Both entities are included in chronic myeloid leukemia - CML - because of the demonstration of the Philadelphia chromosome in the hematopoietic cells of these two groups of patients. Primary or idiopathic thrombocythemia has to be differentiated from CMGM since there is no evidence for malignancy of the granulocytic series.