Neurofibromatosis type one (NF1) is a common genetic disorder affecting 1: 4000 births and is characterized by benign and malignant tumors. Children with NF1 are predisposed to juvenile myelomonocytic leukemia. The Nf1 gene encodes neurofibromin, which can function as a Ras GTPase-activating protein. Neurofibromin deficiency in mice leads to mid-gestation lethality due to cardiovascular defects. We have previously shown that conditional inactivation of Nf1 using Tie2-Cre recapitulates the heart defects seen in Nf1−/-embryos. Tie2-Cre transgenic mice express Cre recombinase in all endothelial cells. Here, we show that Tie2-Cre–mediated deletion of Nf1 also leads to excision of Nf1 in the hematopoietic lineage. Surviving mice exhibit a myeloproliferative disorder similar to juvenile myelomonocytic leukemia seen in NF1 patients. These mice provide a useful model to study neurofibromin deficiency in hematopoiesis. Furthermore, defects in Tie2-Cre–expressing progenitors that result in heart and blood defects suggest that related heart and blood disorders in NF1 and other syndromes represent disorders of the hemangioblast.