Bunworasate et al 1 have recently published a brief report in which they described a transformation of a myelodysplastic syndrome (MDS) in an acute monoblastic leukemia in a patient 6 weeks after the initiation of erythropoietin (EPO) therapy for refractory anemia with ringed sideroblasts. Multiparameter flow cytometric analysis of bone marrow cells demonstrated coexpression of the EPO receptor with CD45 and CD13 on the surface of blasts.The incubation of marrow cells with EPO, compared to without, resulted in 1.3-and 1.6-fold increases, respectively, in tritiated thymidine incorporation and bromodeoxyuridine incorporation into CD13 + cells. The authors concluded that leukemic transformation in patients with EPO may be EPO-dependent and that management should consist of the discontinuation of EPO followed by observation, if clinically feasible.In 1999 we published a paper in which we demonstrated a proangiogenic activity of EPO both in vitro and in vivo. 2 We showed that an endothelial cell line expressed EPO receptors (EPORs) by Western blot analysis. Also EPO stimulated Janus kinase-2 (JAK-2) phosphorylation, cell proliferation, and matrix metalloproteinase-2 production in the endothelial cells and significantly enhanced their differentiation into vascular structures when seeded on Matrigel. In vivo, EPO induced a potent angiogenic response in the chick embryo chorioallantoic membrane (CAM) assay. Accordingly, endothelial cells of the CAM vasculature expressed EPORs and the angiogenic response of CAM blood vessels to EPO was comparable to that elicited by the prototypic angiogenic cyokine fibroblast growth factor-2 (FGF-2). Taken together, these data demonstrated the ability of EPO to interact directly with endothelial cells and to elicit an angiogenic response in vitro and in vivo and thus act as a bona fide direct angiogenic factor.Angiogenesis is a critical event for the progression and prognosis of tumors. 3 Tumor growth consists of an avascular and subsequent vascular phase. Assuming that it is dependent on angiogenesis which, in turn, depends on the release of angiogenic factors, acquisition of an angiogenic capability can be seen as an expression of progression from neoplastic transformation to tumor growth and metastasis. While the role of angiogenesis in the growth, progression and metastasis spread of solid tumors is well established, more recently, it has been demonstrated that the progression of several cancers of hematopoietic lineage, including non-Hodgkin's lymphomas (NHL), lymphoblastic leukemia, B cell chronic lymphocytic leukemia, acute myeloid leukemia, multiple myeloma (MM), and MDS are also clearly related to the degree of angiogenesis. 4 We have previously demonstrated that angiogenesis is correlated with tumor growth (S-phase fraction) in monoclonal gammopathies of undetermined significance (MGUS) and MM grouped according to a pathway of progression, and with progression stages both in B cell NHL and mycosis fungoides. [5][6][7][8] Correspondence Bunworasate et al's paper 1 opens the q...