Good prognosis after imatinib mesylate treatment has been reported if cytogenetic clonal evolution (CE) is the only criterion of accelerated phase (AP) chronic myelogenous leukemia (CML). To evaluate the impact of CE upon imatinib treatment in post-transplant settings, responses and toxicities in the relapsed AP-CE were analyzed in comparison with those in the relapsed chronic phase (CP). Both CP (n=7) and AP-CE patients (n=6) received imatinib mesylate in an oral dose of 400 mg/day. Complete cytogenetic responses were obtained in six patients of each group, CP (86%) and AC-CE (100%), while molecular remission was seen in 43 and 50%, respectively. Granulocytopenia or thrombocytopenia of grade III or more occurred in four (57%) and two (33%) patients with CP and AP-CE, respectively. Nonhematological adverse events were mild and tolerable in both groups and only one (7%) of the 13 patients experienced recurrent graft-versus-host disease after imatinib treatment. Although this is a relatively small group of patients, we suggest that imatinib mesylate should be considered as a front-line treatment for relapsed CML as it showed the high response rate and low toxicity. We also suggest that CE alone is not an important factor in the induction of cytogenetic and molecular remissions in post-transplant relapse.