The IFN-inducible immunity-related p47 GTPase Irgm1 has been linked to Crohn disease as well as susceptibility to tuberculosis. Previously we demonstrated that HSC quiescence and function are aberrant in mice lacking Irgm1. To investigate the molecular basis for these defects, we conducted microarray expression profiling of Irgm1-deficient HSCs. Cell-cycle and IFNresponse genes are up-regulated in Irgm1-/- HSCs, consistent with dysregulated IFN signaling. To test the hypothesis that Irgm1 normally down-regulates IFN signaling in HSCs, we generated Irgm1 -/-Ifngr1-/- and Irgm1-/-Stat1-/- double-knockout animals. Strikingly, hyperproliferation, selfrenewal, and autophagy defects in Irgm1-/- HSCs were normalized in doubleknockout animals. These defects were also abolished in Irgm1-/-Irgm3 -/- double-knockout animals, indicating that Irgm1 may regulate Irgm3 activity. Furthermore, the number of HSCs was reduced in aged Irgm1 -/- animals, suggesting that negative feedback inhibition of IFN signaling by Irgm1 is necessary to prevent hyperproliferation and depletion of the stem cell compartment. Collectively, our results indicate that Irgm1 is a powerful negative regulator of IFN-dependent stimulation in HSCs, with an essential role in preserving HSC number and function. The deleterious effects of excessive IFN signaling may explain how hematologic abnormalities arise in patients with inflammatory conditions.