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The suppression of Ph1-positive hemopoiesis is a major goal in the treatment of CML; in this context the CML patients in blast crisis obtaining a complete clinical remission represent a useful model to investigate the behavior of the Ph1-positive and negative clones during bone marrow repopulation after ablative therapy.Seven CML patients in blast crisis (four lymphoid and three myeloid) who obtained a complete clinical remission after an intensive polychemotherapy treatment were evaluated by cytogenetic and molecular analysis both in blast and remission phases. Standard cytogenetic and Southern techniques were employed; in addition, minimal residual disease status (MRD) was ascertained by amplification (PCR) of the specific bcr-abl chimeric transcripts.After a single cycle of induction, all lymphoid cases displayed a complete restoration of Ph1-negative hemopoiesis; by contrast, one myeloid blast crisis showed a partial suppression of Ph1-positive hemopoiesis only after two cycles of chemotherapy, and in the remaining two cases the hematological remission was indeed a reversion to the chronic phase.Ph1-positive chronic clones present in lymphoid blast crisis showed a higher degree of sensitivity to intensive chemotherapy than those present in the myeloid cases. This observation further suggests that the growth properties of the Ph1-positive clones are highly variable from case to case and probably tend to progress during the time-course of the disease.