An immunomorphometric study was performed on trephine biopsies of the bone marrow in 41 patients with chronic myeloid leukemia (CML) to determine number and size of megakaryocytic precursor cells (pro- and megakaryoblasts). For specific staining, a monoclonal antibody against platelet glycoprotein IIIa (Y2/51) was employed which is applicable on routinely fixed and paraffin embedded tissue. In comparison with control specimens from 15 patients, in CML morphometric analysis revealed an increase in the total amount of megakaryocytes per square and cubic millimeter marrow tissue, but particularly in patients with thrombocythemia. Moreover, a non-disorderly expansion of the megakaryocyte precursor pool was recognizable by showing a relative frequency of pro- and megakaryoblasts in congruence with the normal value. In this context a significant correlation between the counts for Y2/51-positive megakaryocytic elements and promegakaryoblasts with the corresponding platelet values was encountered. The more mature stages of megakaryopoiesis (pro- end megakaryocytes) disclosed a relevant shift to smaller cell forms with rounded cell perimeters and a more compact aspect of their nuclei. Additionally, in 6 patients with CML, evolution into a subacute and manifest (micro)-megakaryoblastic transformation accompanied by myelofibrosis could be demonstrated by a retrospective review of file material.