CD117 protein is expressed by the primitive CD34 positive haemopoietic stem cells and also demonstrated on the blasts of 30-100% of AML cases, but rarely on lymphoblasts. Therefore several investigators have used CD117 expression to exclude lymphoblastic origin of blasts. However, conflicting results exist in the literature. We investigated CD34 and CD117 status at initial presentation of 232 children with acute leukemia. CD34 was commonly expressed in all types of acute leukemias, whereas CD117 molecule seemed to be a more specific marker for leukemia of myeloid origin being demonstrated on > 5% of blasts in 60 out of 73 cases of AML patients, but rarely detected in ALL (9/140 patients). Moreover, co-expression of CD34/CD117 was extremely rare on lymphoblasts with only 3/140 ALL patients demonstrating > 5% co-expression of CD34 and CD117, and therefore we suggest that it should be used in the exclusion of ALL. (C) 2000 Elsevier Science Ltd. All rights reserved.