Imatinib, a BCR-Abl inhibitor, is a successful front-line treatment for chronic myelogenous leukemia (CML). Despite the success of imatinib, multiple mechanisms of resistance remain a problem, including overexpression of Lyn kinase (Lyn) and Bcl-2 family antiapoptotic proteins. Profiling micro-RNA (miRNA) expression in a model of Lyn-mediated imatinib-resistant CML (MYL-R) identified approximately 30 miRNAs whose expression differed >2-fold compared with drug-sensitive MYL cells. In particular, the expression of the miR181 family (a-d) was significantly reduced (∼11- to 25-fold) in MYL-R cells. Incubation of MYL-R cells with a Lyn inhibitor (dasatinib) or nucleofection with Lyn-targeting short interfering RNA increased miR181b and miR181d expression. A similar Lyn-dependent regulation of miR181b and miR181d was observed in imatinib-resistant K562 CML cells. Sequence analysis of potential targets for miR181 regulation predicted myeloid cell leukemia-1 (Mcl-1), a Bcl-2 family member whose expression is increased in MYL-R cells and drug-resistant leukemias. Inhibition of Lyn or rescue of miR181b expression reduced Mcl-1 expression in the MYL-R cells. To further investigate the mechanism of Mcl-1 repression by miR181, a luciferase reporter construct incorporating the Mcl-1 3'-untranslated region was tested. Overexpression of miR181b reduced luciferase activity, whereas these effects were ablated by the mutation of the seed region of the miR181 target site. Finally, stimulation of Lyn expression by 1,25-dihydroxyvitamin D(3) treatment in HL-60 cells, a cell model of acute myelogenous leukemia, decreased miR181b expression and increased Mcl-1 expression. In summary, our results suggest that Lyn-dependent regulation of miR181 is a novel mechanism of regulating Mcl-1 expression and cell survival.