The engraftment of murine hematopoietic stem cells (HSCs) into irradiated mice is thought to be an inefficient process, but has yet to be measured directly. We used two independent strategies to test their engraftment efficiency: one measured competition of unpurified donor bone marrow cells with recipient cells in murine hosts and the other tracked the engraftment of one highly purified stem cell injected per recipient. The results showed that stem cells engrafted with near absolute efficiency. Thus, inefficient engraftment cannot explain the low frequency of permanent reconstitutions observed with pure HSC fractions and instead suggests most initially engrafted cells fail to sustain self-renewal.