A histomorphometric analysis was performed on trephine biopsies of the bone marrow in 55 patients with chronic myeloproliferative disorders (CMPDs) and marked thrombocytosis (platelet count exceeding 600 x 10(9)/l). This study aimed at discriminating primary (essential) thrombocythaemia (PTH) from the various other subtypes of CMPDs presenting with thrombocytosis. Following the diagnostic requirements postulated by the Polycythemia-vera-Study-Group for PTH and polycythaemia vera rubra (P.vera) and the generally accepted criteria for the establishment of chronic myeloid leukaemia (CML) and agnogenic myeloid metaplasia (AMM), our cohort of 55 patients was divided into the following subgroups: CML (16 cases), P.vera (11 cases), AMM (13 cases) and finally PTH (15 cases). Histomorphometric measurements revealed that PTH was distinguishable from the other subtypes of CMPDs with respect to several histological variables: patients with PTH had a normal amount of neutrophilic granulo- and erythrocytopoiesis as well as a non-increased content of reticulin (argyrophilic) fibers in contrast to the findings in CML, P.vera and of course AMM. Moreover, sizes of megakaryocytes and their nuclei were significantly greater in PTH and internalization of haematopoietic cells (emperipolesis) was more frequently encountered in comparison with the other subtypes of CMPDs. Deviation of the circular perimeter of megakaryocyte shape was most prominently expressed in CML and AMM, and consequently generated an increased number of a-nuclear cytoplasmic fragments. In contrast to this feature aberration of the nuclei from a circular outline occurred in a less pronounced way in CML, but was excessive in P.vera, AMM and PTH. Our morphometric evaluation demonstrates that certain histological features may serve as a valuable aid in discriminating PTH from the other occasionally thrombocythaemic subtypes of CMPDs.