In this study we present the applicability of fluorescence in situ hybridisation (FISH) and RT-PCR to detect the minimal residual disease (MRD) in relapsed Ph+ children after donor lymphocyte infusion (DLI) post bone marrow transplantation (BMT). In both patients BCR/ABL fusion was detected and its transcript at the moment of relapse. After the DLI treatment in short time intervals a decreasing number of cells with BCR/ABL fusion were noticed and the expression of the hybrid gene disappeared. These results demonstrate that all the methods presented in this study provide a feasible, rapid and accurate way for the detecting of the minimal residual disease after DLI in Ph positive CML patients.