The Technicon H6000 hematology blood counter is a fully automated analyzer which provides, in addition to conventional hematological blood values (i.e. RBC, PLT, etc.), a differential count of WBCs based on a combination of cytochemistry (peroxidase content) and cell volume analysis. Because of these characteristics, the H6000 quantitizes, as part of the full differential count, the number and percentage of "large unstained cells" (LUCs), i.e. large peroxidase-negative circulating elements with a cell volume above a predetermined threshold established for the lymphocytes of normal subjects. We evaluated the H6000 printouts (scattergrams) of 29 patients with myelodysplastic syndromes and of 12 other patients with transient cytopenias. The most important and constant diagnostic features for myelodysplastic syndromes were the increased proportion of LUCs and, in some cases, the high monocyte count, which are both automatically provided by the instrument.