FMS related tyrosine kinase 3 (FLT-3) is a tyrosine kinase expressed in early hematopoietic precursor cells and has roles in survival, proliferation, and differentiation. Bone marrow expression and mutagenic analysis of FLT-3 in Acute Myeloid Leukemia (AML) patients is well-characterized. However, the levels of circulating FLT-3 in serum have not been previously described. In this study we describe a quantitative electrochemiluminescent immunoassay that detects FLT-3 in human serum. Using this method we find that AML patients have elevated levels of circulating FLT-3 and these levels correlated to the percent blast counts in the bone marrow (BM).