Introduction of a “suicide gene” into donor T-lymphocytes is a promising strategy to prevent severe graft-versus-host disease (GvHD) in the setting of allogeneic blood stem cell transplantation (B-SCT). We initiated a clinical phase I/II-study combining CD34-enriched peripheral B-SCT and Herpes simplex virus thymidine kinase- (HSV-tk) transduced donor T-cell infusions at days 0 and (optional) 60. Before our study was put on hold (after the leukemia cases in the French X-SCID trial) three patients were treated within the protocol each receiving >4x106/kg highly enriched CD34+ and appr. 5x106/kg suicide gene-modified (SGM) donor T-cells. All patients showed fast donor cell engraftment. Patient 2 (without signs of GvHD) received two SGM T-cell infusions and is well >2.5 years thereafter. Patient 1, showed an early (days 11–13) strong increase of SGM cell-counts peaking at 7.2x107/l, but did not develop GvHD. Unexpectedly, his blood was completely cleared from SGM T-lymphocytes a few days later. This patient received a second dose of SGM T-cells (day 65), which vanished within 2 days strongly indicative of their immune rejection. Indeed, MLR data confirmed anti-SGM reactivity of this patient’s PBL possibly related to an HSV reactivation during transplantation. Patient 3 showed a similar early sharp rise in SGM T-cells (days 14–17), in his case associated with acute skin GvHD grade II. Treatment with ganciclovir led to complete resolution of GvHD and rapid disappearance of SGM donor T-cells. Importantly, both patients 1 and 3 developed secondary graft failure at days 156 and 119. In summary, we confirmed previous reports that the use of SGM T-lymphocytes may allow control of acute GVHD. At the same time, we made the worrying observation that early total in vivo depletion of SGM donor T-cells may be associated with an increased risk of transplant rejection. This suggests that minimum numbers of donor CD3+ cells are required post-transplant not only to facilitate engraftment, but also to prevent late rejection.