Haematopoietic stem cells (HSCs) sustain blood production throughout life. HSCs are capable of extensive proliferative expansion, as a single HSC may reconstitute lethally irradiated hosts. In steady-state, HSCs remain largely quiescent and self-renew at a constant low rate, forestalling their exhaustion during adult life^,. Whereas nuclear regulatory factors promoting proliferative programmes of HSCs in vivo and ex vivo have been identified^,,, transcription factors restricting their cycling have remained elusive. Here we report that the zinc-finger repressor Gfi-1 (growth factor independent 1), a cooperating oncogene in lymphoid cells^,, unexpectedly restricts proliferation of HSCs. After loss of Gfi-1, HSCs display elevated proliferation rates as assessed by 5-bromodeoxyuridine incorporation and cell-cycle analysis. Gfi-1^-/- HSCs are functionally compromised in competitive repopulation and serial transplantation assays, and are rapidly out-competed in the bone marrow of mouse chimaeras generated with Gfi-1^-/- embryonic stem cells. Thus, Gfi-1 is essential to restrict HSC proliferation and to preserve HSC functional integrity.