A morphometric analysis was performed on aspirate clots of bone marrow to identify the presence of atypical megakaryocytes after immunohistological staining with a monoclonal antibody against Factor VIII. This study included cases of myelodysplastic syndrome (MDS), myeloproliferative disorder (MPD), aplastic anaemia (AA), idiopathic thrombocytopenic purpura (ITP), chronic myelogenous leukaemia (CML), and control cases free from any haematological disease. Quantitative and qualitative abnormalities of megakaryocytes were assessed using an image analyser and a personal computer to perform a morphometric analysis of the number (per mm2), arrangement (microns), nuclear size (microns2), cell size (microns2), nuclear size/cell size (N/C) ratio, and nuclear contour index (NCI). Micromononuclear megakaryocytes were detected in MDS, while in MPD, large over-mature magakaryocytes were observed to increase in number. In AA, the megakaryocytes decreased dramatically in number without showing any morphological abnormality. In CML, the megakaryocytes also increased without any remarkable morphological abnormality. In ITP, only the NCI increased. The above results show that MDS is characterized by the presence of atypical micromegakaryocytes, while MPD is characterized by atypical large over-mature megakaryocytes.