CD117 is a transmembrane protein receptor encoded by the c-kit proto-oncogene. The CD117 ligand is stem cell factor, an important hematopoietic regulator. CD117 is present on approximately 4% of normal bone marrow mononuclear cells and in acute myelogenous leukemia (AML) and chronic myelogenous leukemia in myeloid blast crisis, but rarely in acute lymphoblastic leukemia (ALL). Initially viewed as a primitive myeloid marker, CD117 has been identified in all FAB subtypes of AML and may predict poor outcome. CD34, a primitive stem cell marker, may also predict poor outcome. The aim of this study was to examine the relationship between CD117 and CD34 expression on leukemic blasts and to determine whether CD117 is related to lymphoid-associated antigen (LAA) expression in AML. Consecutive bone marrow samples were studied from cases of AML (30 cases), myelodysplastic syndromes (MDS) (4 cases), myeloproliferative disorders in blast crisis (MPD-BC) (6 cases), and ALL (5 cases). Cases were diagnosed according to FAB criteria and included M0 (3 cases), M1 (2 cases), M2 (13 cases), M3 (1 case), M4 (6 cases), M5 (3 cases), M6 (1 case), AML NOS (1 case), RAEB (3 cases), and RAEB-T (1 case). CD117 and CD34 were analyzed by multiparameter flow cytometry. Blasts in 10 de novo AML samples were CD117+/CD34+ in 4 cases, CD117+/CD34-in 3 cases, CD117-/CD34+ in 1 case, and CD117-/ CD34- in 2 cases. Blasts in 20 cases of relapsed AML were CD117+/ CD34+ in 13 cases, CD117+/CD34- in 6 cases, and CD117-/CD34+ in 1 case. Blasts in MDS were CD117+/CD34+ in 3 cases, CD117-/ CD34+ in 1 case. Blasts in MPD-BC were CD117+/CD34+ in 4 cases, CD117-/CD34+ in 2 cases. Blasts in ALL were CD117+/CD34+ in 1 case, CD117-/CD34+ in 1 case, CD117-/CD34- in 3 cases. Of 26 cases of CD117+ AML, CD4 was expressed in 15 (58%) cases, CD7 in 7 (27%) cases, and CD2 in 2 (8%) cases. CD117/CD34 expression did not correlate with FAB subtype of AML. CD117 is borne on most leukemic blasts of myeloid origin (in this study, 87% of AML, 80% of MPD-myeloid BC, and 75% of MDS) and does not exclude expression of LAA. Although CD117 is a receptor for stem cell factor, its expression does not appear to correlate with CD34 positivity.