Résumé : A measurement of the tt¯ production cross section in pp collisions at s=7 TeV is presented. The results are based on data corresponding to an integrated luminosity of 2.3 fb−1 collected by the CMS detector at the LHC. Selected events are required to have one isolated, high transverse momentum electron or muon, large missing transverse energy, and hadronic jets, at least one of which must be consistent with having originated from a b quark. The measured cross section is 158.1±2.1(stat.)±10.2(syst.)±3.5(lum.) pb, in agreement with standard model predictions.