The prognosis of relapsed acute leukemia or chronic leukemia in acute blast crisis is poor and new chemotherapeutic regimens could be useful for these patients. Six relapsed acute lymphoblastic leukemia (ALL), nine relapsed acute myeloblastic leukemia (AML), one chronic myelomonocytic leukemia (CMML) and one chronic myeloid leukemia (CML) in acute blast crisis between three to 18 years (median 10 years) received fludarabine, cytarabine, G-CSF and idarubicin (FLAG-IDA) chemotherapy (CT). Five of the AML relapses were after bone marrow transplantation (BMT) and four were recurrent relapses. At the end of the second course only three patients (2 AML, 1 ALL) were in complete remission (CR). Of the three patients in CR, one patient with AML had her first donor lymphocyte transfusion (DLT) on the 7th day of the second FLAG-IDA course and she is disease-free on the 30th month of the second remission. The remaining two patients were transplanted from unrelated donors in a BMT center abroad on the 5th and 8th month of the last remission and both died with BMT-related complications. Out of 25 courses, seven resulted in fatal infections. The regimen was ineffective in B-cell ALL as in acute blastic crisis of CMML and CML. We could not evaluate the remission-inducing effect accurately in most of the patients due to induction failure. FLAG-IDA appears to be a myelotoxic therapy for relapsed or poor risk leukemia in a developing country. It is not cost-effective; dose modifications or a regimen without IDA may be tried if there is an available marrow donor.