Background and Objectives: The microenvironment of acute myelogenousleukemia (AML) is suppressive for immune effector cells. Regulatory Tcells (Tregs) have been recognized as a contributor factor and may berecruited and exploited by leukemic cells to evade immunesurveillance. Themesenchymal stromal cells (MSCs), the second largest population of longlived stem/ progenitor cells in the bone marrow play an important role inimmunomodulation of the microenvironment as they can promotedifferentiation of CD4+CD25+ regulatory T-cell subsets via the indoleamine2,3-dioxygenase (IDO) pathway.The aim of the present work was to evaluate the expression of indoleaminedioxygenase in bone marrow derived MSCs and to study its correlation topercent of Regulatory T cells.Methods: 37 adult bone marrow samples were cultured in appropriateculture medium to isolate MSCs. Successful harvest of MSCs wasdetermined by morphology, plastic adherence and positive expression ofCD271 and CD 105; negative expression of CD34 and CD45 usingflowcytometry. MSCs were examined for IDO expression byimmunocytochemistry using anti-IDO monoclonal antibody. CD4+ CD25+cells (Tregs) were measured in bone marrow samples by flowcytometry.Results: MSCs were successfully isolated from 20 of the 37 bone marrowsamples collected. MSCs showed higher expression of IDO and elevatedTregs percentage in AML patients than in control subjects (p=0.002 andp<0.001 respectively). A positive correlation was found between IDOexpression and Tregs percentage (p value=0.012, r=0.5).Conclusion: In this study we revealed an association between high IDOexpression in MSC and elevated levels of Tregs which has an important rolein the pathogenesis of AML, providing immunosuppressivemicroenvironment.