Low levels of donor chimerism on day 28 after hematopoietic stem cell transplantation are highly predictive of a subsequent rejection of donor cells A recent report on nonmyeloablative transplants has demonstrated that patients who engrafted after transplantation had at least 10% donor cells detectable at this time point. A patient is described who received a nonmyeloablative umbilical cord blood transplant and experienced subsequent full engraftment of donor cells despite the absence of donor chimerism at 4 weeks posttransplantation.