['Quantification of bone marrow (BM) blasts by cytomorphology is essential for the diagnosis of myelodysplastic syndromes (MDS). Owing to its subjectivity and the potential impact of dysplastic features on accurate identification of blast cells, more objective approaches are required, multiparameter flow cytometry (MFC) being a particularly promising approach in this regard. However, no consensus exists about the optimal combination of markers and strategy to be used.', 'BM blast counts from 74 MDS patients were evaluated by morphology versus four different MFC phenotypic criteria: "CD34⁺", "CD34⁺ and/or CD117⁺", "CD34⁺, and/or CD117⁺ HLA-DR⁺", and "CD34⁺ and CD117⁺ HLA-DR⁺ plus CD64⁺ CD14(-/lo) " cells. For each criterium, the percentage of blasts was calculated using either all BM nucleated cells or non-erythroid CD45⁺ cells as denominator. RESULTS.: The number of "CD34⁺ and/or CD117⁺ HLA-DR⁺"cells showed the highest correlation and agreement with morphological counts, only a minor proportion of cases being misclassified by MFC vs. morphology for the >5% and >10% classification thresholds. In turn, a CD34⁺ phenotype was insufficient to correctly identify and quantify blasts. Conversely, usage of non-erythroid BM cells as denominator, or inclusion of "CD34⁺ and/or CD117⁺ HLA-DR⁺ plus CD64⁺ CD14(-lo") cells were both associated with overestimated blast counts.'] Is combined flow cytometric assessment of CD45 , HLA-DR , CD34 , and CD117 expression a useful approach for reliable quantification of blast cells in myelodysplastic syndromes?