Hematopoietic stem cells (HSCs) are rare, ancestral cells that underlie the development, homeostasis, aging, and regeneration of the blood. Here we show that the chromatin-associated protein Ikaros is a crucial self-renewal regulator of the long-term (LT) reconstituting subset of HSCs. Ikaros, and associated family member proteins, are highly expressed in self-renewing populations of stem cells. Ikaros point mutant mice initially develop LT-HSCs with the surface phenotype cKit+Thy1.1(lo)Lin(-/lo)Sca1+Flk2-CD150+ during fetal ontogeny but are unable to maintain this pool, rapidly losing it within two days of embryonic development. A synchronous loss of megakaryocyte/erythrocyte progenitors results, along with a fatal, fetal anemia. At this time, mutation of Ikaros exerts a differentiation defect upon common lymphoid progenitors that cannot be rescued with an ectopic Notch signal in vitro, with hematopoietic cells preferentially committing to the NK lineage. Althoughdispensable for the initial embryonic development of blood, Ikaros is clearly needed for maintenance of this tissue. Achieving successful clinical tissue regeneration necessitates understanding degeneration, and these data provide a striking example by a discrete genetic lesion in the cells underpinning tissue integrity during a pivotal timeframe of organogenesis.