Normal expression of the hematopoietic growth factor receptor FLT3 (STK-1@Flk2) is limited to CD34+ stem/progenitor cells. We have evaluated the expression of FLT3 by RNase protection assay and Western blotting in 161 primary bone marrow (BM) samples from patients with leukemia. FLT3 RNA was found to be expressed at a higher level than in normal BM controls in 33 of 33 B-lineage acute leukemias, 11 of 12 acute myeloid leukemias (AMLs), and 3 of 11 T-cell acute leukemias (T-ALLs). Expression of FLT3 RNA was also observed in some cases of blast crisis CML. The FLT3 signal resulted from expression on the leukemic blasts, and was not caused by increased FLT3 expression on normal CD34+ stem/progenitor cells in the leukemic samples. To determine if FLT3 protein was also overexpressed, proteins were extracted from leukemic BM samples and screened by Western blotting with anti-FLT3 antisera. FLT3 protein was not detected in normal BM controls, but was found in 14 of 14 B-lineage ALLs, 36 of 41 AMLs, and 1 of 4 T-ALLs. Stimulation of patient samples with FLT3 ligand resulted in autophosphorylation of the FLT3 receptor, suggesting the receptor is functional in these cells. These data show that FLT3 RNA and protein are aberrantly expressed by AML and ALL cells in that CD34 expression and FLT3 expression are no longer synchronous, and suggest the possibility that overexpression of FLT3 could play a role in the survival and/or proliferation of malignant clones in acute myeloid and lymphoid leukemias.