Kinetics of BCR-ABL transcript levels were determined in 19 patients with chronic myeloid leukemia (CML) treated with imatinib for chronic (CP) or accelerated phase (AP). Patients could be divided into three groups with: (1) a sharp and sustained decrease in BCR-ABL transcript level reaching 0.1-0.002% (only CP); (2) an early BCR-ABL overexpression up to 2500% (only AP); and (3) a stable trend with BCR-ABL values between 10 and 100% (CP, AP). In group 1, relapses were not developed within the follow-up; in group 2, patients progressed to blast crisis; in group 3, BCR-ABL overexpression appeared after 12 months in some patients and disease relapses were found 2-16 weeks later. It is summarized that BCR-ABL transcript kinetics clearly characterize responses to imatinib treatment and are highly predictive for disease progression.