The success of allogeneic stem cell transplantation (allo-SCT) in children is mainly affected by relapse or graft rejection. We have recently shown in a study of 55 patients with acute leukemias (ALL 21, AML 20 and MDS 14), that patients who demonstrate increase amounts of autologous marrow repopulation (increasing mixed chimerism) have a significantly enhanced risk of relapse (P < 0. 0001). Based on these findings, we asked whether post-transplant relapse can be prevented by withdrawal of immunosuppression and/or by donor lymphocyte infusion (DLI). We describe the results of a pilot study where adoptive immunotherapy was used to treat 12 patients (five ALL, three AML, four MDS) who showed increasing mixed chimerism (MC) post-transplant. A response to immunotherapy, defined as the re-establishment of complete chimerism (CC) and continuous complete remission (CCR), was achieved in four patients (two ALL, two AML) following withdrawal of CsA and in a further six patients (three ALL, three MDS) after additional DLI. One ALL patient, who initially responded to DLI, developed severe GVHD that required further immunosuppression. GVHD was controlled but this patient subsequently relapsed. Another patient with ALL became a CC but developed an isolated relapse in the bone marrow 260 days later. One patient with MDS developed severe GVHD after DLI and died. Two children (one AML and one MDS) did not show any response to interventional treatment and died due to relapse. Of the 12 patients treated, seven remain in CCR at a median follow-up of 747 days (range 351-1109 days). In summary, these results provide evidence that increasing MC can be used to guide adoptive immunotherapy strategies and that these treatment modalities can be used to prevent relapse in children with acute leukemias or MDS after allo-SCT.