Morphological features of megakaryocytes were studied in 31 patients with myelodysplastic syndrome. Mean nuclear size (N), mean cytoplasmic size (C) and mean N/C ratio were 586 +/- 105 mu 2, 1,391 +/- 327 mu 2 and 0.42 +/- 0.04, respectively. Cell sizes were found to be reduced in 12 patients compared to control subjects. Atypical megakaryocytes were frequently found in patients with RAEB and micromegakaryocytes were also often found in patients with CMML. Mean platelet volume was 0.9 +/- 1.6 fl and the giant platelets were seen in one patient with CMML. In the patients with presence of more than 10% of atypical megakaryocytes, template Ivy bleeding time prolonged and epinephrine- and collagen-induced aggregation decreased. We believe that reduction of megakaryocytes' size and appearance of micromegakryocytes would be helpful in diagnosis of MDS.