The aim of the study was an estimation of the histomorphometry of megakaryocytes (MK) in bone marrow in selected myeloproliferative and lymphoproliferative diseases. Bone marrow specimens were obtained by trephine biopsies from 41 patients with polycythaemia rubra vera (PV), idiopathic myelofibrosis (MF), chronic lymphocytic leukaemia (CLL), hairy cell leukaemia (HCL) and diffuse large B-cell lymphoma (L). Morphometric evaluation was performed using a standard program set MicroImage (OLYMPUS). The greatest number of typical nucleated MK, "naked" nuclei, anucleated cytoplasmic fragments and the largest area were found in PV. The circular deviation factor of MK and their nuclei increased in all cases. The greatest number of clusters was observed in PV and HCL. A significant increase in the number of dysplastic and "naked" nuclei of MK was noted in all selected haematological diseases. The presence of neoplastic cells in bone marrow increased the morphological changes in MK. Quantitative and morphometrical significant differentiation of MK in separate microscopic field in the same slides confirms the necessity of performing trephine biopsies in each patient with haematological disorders.