Summary:The aim of this study was to reduce the rate of graft failure after HLA non-identical stem cell transplantation by using G-CSF mobilized CD34 ؉ peripheral blood progenitor cells (PBPC), either in combination with bone marrow or as single grafts. To prevent GVHD, PBPC were highly purified, resulting in a 5 to 6 log T cell depletion. In additon to T cell depletion no further GVHD prophylaxis was used. We transplanted 23 pediatric patients with life-threatening malignant or non-malignant hematological disorders, who had no available matched donor. Engraftment was obtained in 18 of 21 evaluable patients. Five patients developed acute GVHD of grade II and III, which became chronic in four cases and was fatal in four. The use of highly purified PBPC allowed the exact quantification of residual T cells in the grafts and a strict correlation between the residual T cell load and the development of GVHD was observed: patients with GVHD had received numbers of T cells between 8 and 20 ؋ 10 4 /kg, whereas patients without GVHD were grafted with T cell numbers ranging from 0.7 to 6.0 ؋ 10 4 /kg. We therefore clearly demonstrate that a residual T cell content of Ͻ5 ؋ 10 4 /kg is safe for prevention of GVHD after HLA non-identical PBPC transplantation in children. Keywords: HLA non-identical transplantation; peripheral blood progenitor cells; grafted T cell number; GVHD HLA haplo-identical BMT with T cell depletion (TCD) has been used successfully to treat children with primary immunodeficiency disorders. [1][2][3] In patients with other diseases, this transplant approach has frequently failed because of graft rejection. [4][5][6] Graft failure may result from the profound T cell depletion required to prevent GVHD, with the ensuing loss of the graft-enhancing effect mediated by T cells. 7,8 More intensive immunosuppressive conditioning to counterbalance this disadvantage has usually had limited success because of significant complications from toxicity. Less vigorously T cell-depleted grafts carry the risk of severe GVHD. From animal models it is clear that engraftment may also be improved by increasing the number of grafted cells, either using unmanipulated bone marrow, taking advantage of the effect of alloreactive T cells or, as shown in recent studies, by using high numbers of highly purified T cell-depleted stem cells. 9 The latter approach was used by Aversa et al to transplant patients with leukemia from HLA non-identical family donors using either a combination of purified peripheral blood progenitor cells (PBPC) and T cell-depleted marrow or, in more recent patients, PBPC alone. 10-12 A remarkably high engraftment rate of more than 90% was observed.Here we report results of a pilot study in pediatric patients, where we explored a comparable transplant approach. Like the findings of Aversa et al, we observed a high engraftment rate of 86%. In contrast, GVHD developed in the initial patients, but was completely preventable in patients receiving grafts containing fewer T cells than 5 ϫ 10 4 /kg. Patients and meth...