Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal stem cell disorders that are characterized by morphology identifying dysplastic changes in one or more cell lineages, peripheral blood cytopenias and a propensity to evolve into secondary acute myeloid leukemia (AML). CD34 is commonly expressed in all types of childhood leukemias, whereas CD 117 is a reliable and specific marker to detect leukemia cells committed to myeloid lineage. Co-expression of CD34/CD117 may strongly suggest the diagnosis of AML (Rytting ME. Pediatric myelodysplastic syndromes. Curr hematot Rep 2004;3(3):173-7. May; Uckan D, Hicsonmez G, Yetgin S, Gurgey A, Cetin M, Karaagaoglu E, et at. CD34/CD117 co-expression in childhood acute leukemia. Leukemia Res 2000;24:201-6.). We describe the case of a 22 month-old-girl with MDS and Down syndrome who was presented with severe anemia and thrombocytosis at diagnosis, transformed into AML-M7. In our patient, CD34 and CD 117 markers were positive on the blast cells of the BM 6 months before the chemotherapy decision. As the disease progressed, CD34/117 co-existence was increased and MDS transformed into AML. As a result, an increase in CD34 and CD117 positivity of the BM blast cells may be associated with a higher risk of leukemic transformation. (c) 2005 Elsevier Ltd. All rights reserved.