Background. Current World Health Organization classification of myelodysplastic syndromes is based on morphological evaluation of marrow dysplasia. In clinical practice, the reproducibility for recognition of dysplasia is usually poor especially in cases that lack specific markers such as ring sideroblasts and clonal cytogenetic abnormalities. Design and Methods. We aimed to develop and validate a flow cytometric score for the diagnosis of myelodysplastic syndrome. Four reproducible parameters were analyzed: CD34+ myeloblast-related and B-progenitor-related cluster size (defined by CD45 expression and side scatter characteristics on CD34+ marrow cells), myeloblast CD45 expression and granulocyte side scatter value. Study comprised a "learning cohort" (n=538) to define the score and a "validation cohort" (n=259) to confirm its diagnostic value. Results. With respect to non-clonal cytopenias, patients with myelodysplastic syndrome presented increased myeloblast-related cluster size, decreased B-progenitor-related cluster size, aberrant CD45 expression and reduced granulocyte side scatter (P<.001). To define the flow cytometric score, these four parameters were combined in a regression model and the weight for each variable was estimated based on coefficients from that model. In the learning cohort a correct diagnosis of myelodysplastic syndrome was formulated in 198/281 cases (sensitivity 70%), while18 false-positive results were noticed among 257 controls (specificity 93%). Sixty-five percent of patients without specific markers of dysplasia (ring sideroblasts and clonal cytogenetic abnormalities) were correctly classified. A high flow cytometric score value was associated with multilineage dysplasia (P=.001), transfusion-dependency (P=.02), and poor risk-cytogenetics (P=.04). Comparable sensitivity and specificity were obtained in the validation cohort (69% and 92%, respectively). The likelihood ratio of the flow cytometric score was 10.Conclusions. Flow cytometric score may help establish the diagnosis of myelodysplastic syndrome, especially when morphology and cytogenetics are indeterminate.