A measurement of the t (t) over bar production cross section in proton-proton collisions at a centre-of-mass energy of 7 TeV has been performed at the LHC with the CMS detector. The analysis uses a data sample corresponding to an integrated luminosity of 36 pb(-1) and is based on the reconstruction of the final state with one isolated, high transverse-momentum electron or muon and three or more hadronic jets. The kinematic properties of the events are used to separate the t (t) over bar signal from W+jets and QCD multijet background events. The measured cross section is 173(-32)(+39) (stat. + syst.) pb, consistent with standard model expectations.