Time to progression by molecular response to imatinib, which was determined by longitudinal analysis of MRD levels throughout study treatment. All patients had detectable bcr-abl transcripts at study entry. Median TTP in patients who were found to convert to PCR negativity soon after initiation of imatinib was significantly longer than in patients who remained PCR positive (28.6 months versus 3.6 months; P < .001). Estimated progression-free survival rate for MRD responders was 91% ± 9% at 12 months and 68% ± 21% at 24 months compared with 8% ± 7% at 13 months for nonresponders.