The W-boson helicity fractions in top-quark decays are measured with tt̄ events in the lepton+jets final state, using proton-proton collisions at a centre-of-mass energy of 7TeV, collected in 2011 with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 5.0 fb -1. The measured fractions of longitudinal, left-, and right-handed helicity are F0 = 0:682±0:030 (stat.)± 0:033 (syst.), F= 0:310 ±0:022 (stat.)± 0:022 (syst.), and F= 0:008± 0:012 (stat.) ± 0:014 (syst.), consistent with the standard model predictions. The measured fractions are used to probe the existence of anomalous Wtb couplings. Exclusion limits on the real components of the anomalous couplings gL; gR are also derived.