Two male patients with Philadelphia-chromosome (Ph+) chronic myelogenous leukemia (CML) underwent allogeneic bone marrow transplantation (ABMT) in the first chronic phase after busulfan treatment. In both cases, the donor was a sister, and engrafting was demonstrated by chromosome analyses which showed only donor cells in the BM. Cytogenetic relapse occurred 29 and 30 months after ABMT, respectively, when host cells reappeared: in both cases, the Ph and additional anomalies typical of the blastic phase of CML were evident. We then monitored the chromosome picture for 52 and 39 months, respectively: no striking evolution occurred, and cells with the Ph and additional anomalies persisted together with donor cells, which were a minority in the first patient and a great majority in the second throughout the observation period. A clinical relapse was observed in the first patient, but the disease never progressed to a blastic phase, whereas the second patient has not relapsed 7 years after ABMT. We reviewed data from the literature on cytogenetic relapse after ABMT in CML without clinical relapse, especially the 12 patients in whom cytogenetic relapse included chromosome anomalies in addition to the Ph, as in our patients. We suggest that graft-versus-leukemia (GVL) reactions in such patients are able to arrest progression of the leukemic blastic clone and prevent a possible relapse in blastic phase.