peer reviewed ; This retrospectivemulticenter study in patientswith chronic myeloid leukemia in chronic phasewas undertaken to confirm the clinical relevance of imatinib plasma concentrations monitoring in daily practice. Forty-one patients, with 47 imatinib plasma measurements, were analyzed during treatment with imatinib given at a fixed 400mg daily dose. A significant inverse relationship of imatinib concentration with the patients' weight was observed (Pearson's test: p=0.02,R2=0.1). More interestingly, patientswith poor response (switched to another tyrosine kinase inhibitor because of imatinib failure, or because of disease progression after an initial response) displayed a significantly lower mean imatinib concentration as compared to patients maintained on imatinib (822 ng/mL vs 1099 ng/mL; Student's t-test, p=0.04). Failure or disease progression occurred more often in patients in the lowest quartile of imatinib concentrations compared to patients in the highest quartile (p = 0.02, logrank test). No correlation could be established with other biological or clinical parameter, including complete cytogenic response and majormolecular response. In conclusion: in patients treatedwith imatinib at a fixed daily dose of 400 mg, imatinib plasma concentrations decreased with increasing body weight and were lower in patients switched to another tyrosine kinase inhibitor due to imatinib failure. Systematic determination of imatinib plasma trough levels should be encouraged in such patients.