An allogeneic sex-mismatched BMT which was performed in a male patient with BCR-ABL-positive ALL in second hematological and central nervous system relapse resulted in a CR for 12 months. After BMT, the patient was closely monitored with reverse transcription (RT)-PCR. One month before a third relapse RT-PCR became positive. During relapse G-CSF was administered. It specifically stimulated the donor-derived myelopoiesis and led to the stabilization of the disease for 8 months. Fluorescence in situ hybridization analyses of individual cell populations revealed that during the whole course of G-CSF administration granulocytes, CD4+, CD8+ and CD34+/CD10- cells were of female (donor) origin and only the CD34+/CD10+ cells which represented the leukemic blasts, were of male (host) origin.