Gradual allograft rejection after initial good engraftment may occur with simultaneous autologous reconstitution particularly in patients receiving nonmyeloablative conditioning. Careful post-transplant follow-up of the chimerism status can reveal these cases early on, when the immunological balance may still be shifted to the donor cells. We describe two children with nonmalignant diseases, in whom imminent rejection of their sibling allografts was prevented with donor lymphocyte transfusions (DLT). DLT dosing and timing need to be individually guided by monitoring of the chimerism status.