A 32-yr-old man with the chronic phase of chronic myeloid leukemia (CML-CP) was treated with imatinib mesylate for 6 mo. The real-time quantitative reverse transcription PCR ratio for BCR/ABL in blood mRNA (BCR/ABL RT-QPCR) decreased from an initial value of 0.0159 to a low value of 0.0012 after 3 mo, indicating complete hematologic response. During the next 3 mo, the patient progressed to a promyelocytic blast crisis, displaying leukemic cells containing both BCR/ABL and PML/RARalpha chimeric mRNAs. Complete remission was achieved by therapy with all-trans retinoic acid (ATRA) and high-dose imatinib mesylate. Using retrospective PML/RARalpha RT-QPCR with a bone marrow specimen obtained at the initial diagnosis of CML-CP, we quantified the mRNA ratio as 0.000321, suggesting that the clonal evolution of PML/RARalpha translocation occurred early in the CML-CP.