Hypoxia-induced signaling is important for normal and malignant hematopoiesis. The transcription factor hypoxia-inducible factor-1α (HIF-1α) has a crucial role in quiescence and self-renewal of hematopoietic stem cells (HSCs), as well as leukemia-initiating cells (LICs) of acute myeloid leukemia and chronic myeloid leukemia. We have investigated the effect of HIF-1α loss on the phenotype and biology of FLT-3(ITD)-induced myeloproliferative neoplasm (MPN). Using transgenic mouse models, we show that deletion of HIF-1α leads to an enhanced MPN phenotype reflected by an increased number of white blood cells, more severe splenomegaly and decreased survival. The proliferative effect of HIF-1α loss is cell intrinsic as shown by transplantation into recipient mice. HSC loss and organ-specific changes in the number and percentage of long-term HSCs were the most pronounced effects on a cellular level after HIF-1α deletion. Furthermore, we found a metabolic hyperactivation of malignant cells in the spleen upon loss of HIF-1α. Some of our findings are in contrary to what has been previously described for the role of HIF-1α in other myeloid hematologic malignancies and question the potential of HIF-1α as a therapeutic target.