ABCB1/P-glycoprotein (Pgp) and ABCG2/BCRP overexpression have been described as related to imatinib resistance in chronic myeloid leukemia (CML). We showed in CML cells from 55 patients that Pgp activity was more frequently detected than BCRP activity (p=0.0074). Imatinib-induced Crkl phosphorylated protein (pCrkl) reduction was more pronounced in K562 (Pgp-negative) than in K562-Lucena (Pgp-positive) CML cell line. Expressive pCrkl reduction levels after in vitro imatinib treatment was observed in samples from patients exhibiting lower Pgp activity levels compared with patients exhibiting higher Pgp activity levels (p=0.0045). Pgp activity in association with pCrkl reduction levels might help to distinguish between imatinib-resistant and imatinib-sensitive CML cells.