Fresh frozen bone marrow biopsies were evaluated immunohistochemically, applying monoclonal antibodies against CD31, CD34, VEGFR-2 and CD133, a novel marker identifying human endothelial progenitor cells (EPCs). Specimens of 51 patients diagnosed with MDS were compared with 16 AML and 18 controls. The percentage of CD34 expressing cells was increased and CD31 expression was decreased in advanced stages of MDS compared with normal BM. VEGFR-2 expression was also raised in MDS. Here we show for the first time that increased numbers of CD133 positive cells are present in the majority of MDS patients. Additionally, those cells occasionally seem to contribute to capillary forming units in bone marrow.