E21R is a modified granulocyte macrophage–colony-stimulating factor (GM-CSF) protein which results in antagonism of GM-CSF function via selective binding to the GM-CSF receptor complex. Juvenile chronic myelomonocytic leukemia (JMML) is a rare leukemia where spontaneous proliferation of myeloid and monocytic precursors in patients' bone marrow cultures is dependent on GM-CSF. For patients who progress after systemic chemotherapy, there are no effective therapies. In vitro and in vivo studies in an animal model demonstrating that E21R exerts an antileukemic action prompted us to consider its potential utility in a child with end-stage JMML. E21R was well-tolerated during the 3 courses of subcutaneous treatment. A clear in vivo efficacy was observed after 2 courses of E21R but the disease appeared completely refractory during the third course. This novel therapeutic approach clearly deserves further evaluation in JMML.