Résumé : A first measurement of the polarization of W bosons with large transverse momenta in pp collisions is presented. The measurement is based on 36pb⊃-1 of data recorded at √s=7TeV by the CMS detector at the LHC. The left-handed, right-handed, and longitudinal polarization fractions (f L, fR, and f0, respectively) of W bosons with transverse momenta larger than 50GeV are determined by using decays to both electrons and muons. The muon final state yields the most precise measurement: (fL-fR)⊃-=0.240±0.036(stat)±0.031(syst) and f0-=0.183±0.087(stat)±0.123(syst) for negatively charged W bosons and (fL-fR)⊃+=0.310±0.036(stat) ±0.017(syst) and f0+=0.171±0.085(stat)±0.099(syst) for positively charged W bosons. This establishes, for the first time, that W bosons produced in pp collisions with large transverse momenta are predominantly left-handed, as expected in the standard model.