OBJECTIVES: An analysis recently evaluated hematologic responses among 126 Iraqi Ph+ CML-CP patients who were switched from Glivec (the beta crystalline form of imatinib mesylate [IM]), to an alpha crystalline form, generic copy of imatinib (IMgc).  At the time of IMgc switch, patients had received IM for 50 months and were at least in complete hematologic response (CHR).  Three months post-switch, 18 (14%) and 4 (3%) patients progressed to accelerated phase (AP) and blast crisis (BC), respectively. Six months post-switch, an additional 20 (16%) lost hematologic response. A previously-published CML Markov model was adapted to compare over 50 years the projected life-years (LYs), progression-free life-years (PFLY), and quality-adjusted life-years (QALYs) of IM patients switched vs. not switched to IMgc.  METHODS: Patients entered the model after 50 months of IM therapy. At that time, based on the IRIS trial results, patients were assumed to be distributed in CHR (4.7%), partial (6.5%) and complete (88.9%) cytogenetic response. Patients remaining on IM transitioned within these 3 responses levels and no hematologic response, AP, BC, and death according to the original model probabilities. For patients switched to IMgc, transition rates were based exclusively on rates observed in the Iraqi study (Scenario 1) or on the Iraqi study for the first 6 months and thereafter on the original model (Scenario 2). Utilities were from the original model.   RESULTS: Patients remaining on IM were predicted to experience 15.71 LYs, 14.51 PFLYs, and 13.44 QALYs. Corresponding numbers for patients switched to IMgc were 2.42 LYs, 1.13 PFLYs, 1.39 QALYs (in Scenario 1) and 11.44 LYs, 10.19 PFLYs, and 9.57 QALYs (in Scenario 2).  Results were also sensitive to response distribution at model entry.  CONCLUSIONS: Switching from IM to an IMgc that does not have the same properties may result in substantial loss of LYs, PFLYs, and QALYs.